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When Harry gets his wand, Ollivander's magical measuring tape first takes all kinds of ridiculous measurements on him, like the distance between his nostrils, from eye to ear or something like that.

Were these measurements actually used for anything, like narrowing down the selection of wands, or was it just to distract/entertain the customer while the real choice/guessing of wands was done based on Ollivander's experience and the responses to the previously tested wands? Or did it previously have a purpose and now that he's more experienced in picking wands, he does it out of habit?

Since we are dealing with magic, the tape measure may have been gathering something more than physical measurements.

Many wandmakers simply match the wand length to the size of the witch or wizard who will use it, but this is a crude measure, and fails to take into account many other, important considerations. In my experience, longer wands might suit taller wizards, but they tend to be drawn to bigger personalities, and those of a more spacious and dramatic style of magic. Neater wands favour more elegant and refined spell-casting. However, no single aspect of wand composition should be considered in isolation of all the others, and the type of wood, the core and the flexibility may either counterbalance or enhance the attributes of the wand’s length.

Most wands will be in the range of between nine and fourteen inches. While I have sold extremely short wands (eight inches and under) and very long wands (over fifteen inches), these are exceptionally rare. In the latter case, a physical peculiarity demanded the excessive wand length. However, abnormally short wands usually select those in whose character something is lacking, rather than because they are physically undersized (many small witches and wizards are chosen by longer wands).

_{Wand Lengths & Flexibility by J.K. Rowling on Pottermore}

Mr. Ollivander really likes to match the wand's and wizard's personality. So while the tape measure may have been seeing if there was any physical peculiarity Harry possessed that would require a certain sized wand it could have been measuring his personality in some magical way given that Mr. Ollivander doesn't pay much attention to it (he is already looking for wands while the measuring is still taking place).

I should add that this is just a theory as there is limited information we have on wand lengths in general and we only see Harry getting measured for a wand. Wand sizes are in pretty tight range (5" with 1/4" increments) so even an 11 year-old would be able to handle any sized wand (with the aforementioned physical peculiarity being the exception).

I would argue that the measurements are probably mostly meaningless. Consider that most people buy their wands when they are eleven years old. Consider that most people also change most of their body measurements after they are eleven years old, and in significant amounts. As such, any measurements Ollivander takes would eventually be obsolete, and often in a very short period of time.

Throughout the series we never find anyone buying a new wand because their original wand "no longer fits". In fact, the only person mentioned to get a new wand at all (aside for the cases where the old wand was lost or broken) is Charlie Weasley. It is not stated what his reason for getting a new wand was, but, judging by the state it was in when Ron got it, it is quite possible that it was not in peak condition, which would easily explain the desire to replace it.

In fact, we find examples of wizards still using their original wands many years, and even decades, after buying them, when presumably their measurements have changed significantly. For instance, Voldemort was still using his original wand of yew and phoenix feather up until the moment he procured the Elder Wand.

Additionally, there doesn't seem to be much correlation between the measurements and the wand choices. We are privy to the descriptions of four of the wands that Harry tried:

• 
  

  Beechwood and Dragon Heartstring. Nine inches. Nice and flexible.

  
  




• 
  

  Maple and Phoenix feather. Seven inches. Quite whippy.

  
  




• 
  

  ebony and unicorn hair, eight and a half inches, springy.

  
  




• 
  

  holly and phoenix feather, eleven inches, nice and supple.

  
  

As we can see, these wands have various different cores, wood type, and flexibility. And the longest one is more than 1.5 times the length of the shortest one. And, for all we know, the dozens of other wands that he tried may have had even more variation.

On the other hand, we do find some amount of correlation between wand length and wizard height:

• 
  Hagrid, a half-giant, has a sixteen inch wand.
  
  

  
  

  "Rubeus! Rubeus Hagrid! How nice to see you again.... Oak, sixteen inches, rather bendy, wasn't it?"

  
  

  
  


• 
  Voldemort, who is tall¹, has a thirteen and a half inch wand.
  
  

  
  

  "Yes, thirteen-and-a-half inches. Yew.

  
  

  
  


• 
  Bellatrix, who is taller than Harry², has a twelve and three quarters inch wand.
  
  

  
  

  "Twelve-and-three-quarter inches. Unyielding. This wand belonged to Bellatrix Lestrange.

  
  

  
  


• 
  Peter Pettigrew, who was short³, had a nine and a quarter inch wand.
  
  

  
  

  "Chestnut and dragon heartstring. Nine-and-a-quarter inches. Brittle. I was forced to make this shortly after my kidnapping, for Peter Pettigrew.

  
  

  
  


• 
  James Potter, who is about the same height as Harry⁴, also has an eleven inch wand.
  
  

  
  

  "Your father, on the other hand, favored a mahogany wand. Eleven inches.

  
  

  
  


• 
  Ron, who is tall⁵, has a fourteen inch wand.
  
  

  
  

  "Look at this," said Ron, pulling a long thin box out of a bag and opening it. "Brand-new wand. Fourteen inches, willow, containing one unicorn tail-hair.

  
  

  
  


• 
  Cedric Diggory, who is tall⁶, has a twelve and a quarter inch wand.
  
  

  
  

  Twelve and a quarter inches... ash... pleasantly springy.

  
  

  
  


• 
  Mrs. Cattermole, who is short⁷, has an eight and three quarters inch wand.
  
  

  
  

  "Eight-and-three-quarter inches, cherry, unicorn-hair core.

  
  

  
  

There are only a few other people whose wand length we know, but their lengths may be less correlative with their height:

• 
  Draco Malfoy, who is tall⁸, only has a ten inch wand.
  
  

  
  

  "Hawthorn and unicorn hair. Ten inches precisely. Reasonably springy. This was the wand of Draco Malfoy."

  
  

  
  


• 
  Viktor Krum, who is tall⁹, only has a ten and a quarter inch wand.
  
  

  
  

  "Rather thicker than one usually sees... quite rigid... ten and a quarter inches... Avis!"

  
  

  
  


• 
  Fleur Delacour, who is tall¹⁰, only has a nine and a half inch wand.
  
  

  
  

  "Yes," he said quietly, "nine and a half inches... inflexible... rosewood... and containing... dear me..."

  
  

  
  


• 
  Lily Potter had a ten and a quarter inch wand, but I don't think we
  know how tall she was.
  
  

  
  

  Ten and a quarter inches long, swishy, made of willow.

  
  

  
  

Even if these numbers seem pretty correlative, we have to keep in mind that, as mentioned above, the heights would have been very different at the time these people bought their wands, so this doesn't necessarily mean much. It could be a coincidence, and/or the author may have deliberately made a connection between the person's final height and wand length.

In any event, this would only account for the height related measurements. Head circumference and distance between nostrils would still be entirely unexplained. I think it is more likely that the measurements are not really that important. Perhaps Ollivander just takes them to make it seem like there's a real process going on, when in reality he is basing his wand suggestions on who-knows-what.

1. But then, through the mist in front of him, he saw, with an icy surge of terror, the dark outline of a man, tall and skeletally thin, rising slowly from inside the cauldron.
   

2. She was taller than he was, her long black hair rippling down her back, her heavily lidded eyes disdainful as they rested upon him; but then she spoke, and he heard Hermione through Bellatrix's low voice.
   

3. He was a very short man, hardly taller than Harry and Hermione.
   

4. James’s hair
   stuck up at the back exactly as Harry’s did, his hands
   could have been Harry’s, and Harry could tell that
   when James stood up, they would be within an inch
   of each other’s heights.
   

5. "I'm tall," said Ron inconsequentially.

6. Two tall figures were silhouetted against the starry sky on the other side of the hilltop.

7. A small woman stood up; she was trembling from head to foot.

8. Hulking boys though they were, they looked oddly lonely without the tall, pale figure of Malfoy between them, bossing them around.

9. But Krum glowered at him, and Harry, somehow struck anew by how tall Krum was, elaborated.

10. She was tall and willowy with long blonde hair and appeared to emanate a faint, silvery glow.

He appears to use them but no explicit mention is made, see the relevant quote below taken from the Pottermore article Harry's curious wand.

‘Hmmm,’ said Mr Ollivander, giving Hagrid a piercing look. ‘Well, now – Mr Potter. Let me see.’ He pulled a long tape measure with silver markings out of his pocket. ‘Which is your wand arm?’

‘Er – well, I’m right-handed,’ said Harry.

‘Hold out your arm. That’s it.’ He measured Harry from shoulder to finger, then wrist to elbow, shoulder to floor, knee to armpit and round his head. As he measured, he said, ‘Every Ollivander wand has a core of a powerful magical substance, Mr Potter. We use unicorn hairs, phoenix tail feathers and the heartstrings of dragons. No two Ollivander wands are the same, just as no two unicorns, dragons or phoenixes are quite the same. And of course, you will never get such good results with another wizard’s wand.’

Harry suddenly realised that the tape measure, which was measuring between his nostrils, was doing this on its own. Mr Ollivander was flitting around the shelves, taking down boxes.

‘That will do,’ he said, and the tape measure crumpled into a heap on the floor. ‘Right then, Mr Potter. Try this one. Beechwood and dragon heartstring. Nine inches. Nice and flexible. Just take it and give it a wave.’

Harry took the wand and (feeling foolish) waved it around a bit, but Mr Ollivander snatched it out of his hand almost at once.

_{Harry Potter and the Philosopher's Stone}

From my reading of the passage it appears as if he is measuring Harry's arm so get the correct measurements for the size of the wand. This would be why he asks Harry what his wand arm is rather than just measuring everything. The head could then be measured to see where certain features are for "aiming" the wand potentially. Note that the only measurements mentioned are right side of the body and head.

It's also worth noting that Ollivander tells the tape to stop measuring once he's found the first possible wand suggesting he doesn't need the measurements anymore and he has what he needs.

From a real world perspective the same is true of archers; bows come in different sizes and so measurements need to be taken to see what the correct size is. For example, see this article on choosing the right size recurve bow and what to measure i.e. finding one's draw length.