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.
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.
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.
He was a very short man, hardly taller than Harry and Hermione.
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.
"I'm tall," said Ron inconsequentially.
Two tall figures were silhouetted against the starry sky on the other side of the hilltop.
A small woman stood up; she was trembling from head to foot.
Hulking boys though they were, they looked oddly lonely without the tall, pale figure of Malfoy between them, bossing them around.
But Krum glowered at him, and Harry, somehow struck anew by how tall Krum was, elaborated.
She was tall and willowy with long blonde hair and appeared to emanate a faint, silvery glow.