## Length

As far as I can find, a wand's length has nothing to do with magical ability. The only correlation between the wand and the wizard here is *possibly* height or personality:

> All wands have their individual lengths and may also be some correlation between a person's size and the lengths of their wands. Rubeus Hagrid, who is half-giant, owned one of the longest known wand; it was sixteen inches long and made of oak.

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> According to Garrick Ollivander, matching a wand to a wizard solely by height is a crude measure. Long wands tend to suit those with big personalities, of a more spacious and dramatic style of magic.  
> <sub>— [Wandlore](http://harrypotter.wikia.com/wiki/Wandlore), Harry Potter Wikia</sub>

## 'Neatness'
I've found *one* reference to a correlation between wand 'neatness' (whatever that is, in a wand) and how refined one's magic is:

> Neater wands favour elegant and refined spell-casting.  
> <sub>— [Wandlore](http://harrypotter.wikia.com/wiki/Wandlore), Harry Potter Wikia</sub>

## Woods
There is some more indication that the type of wood a wand is made from has some correlation with the wizard's calibre:

> [Acacia] "A very unusual wand wood, which I have found creates tricky wands that often refuse to produce magic for any but their owner, and also withhold their best effects from all but those most gifted."  
> <sub>*From Ollivander's notes on wand woods* — [Wand Woods](http://pottermore.wikia.com/wiki/Wand_Woods), Pottermore Wikia</sub>

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> The proper owner of the aspen wand is often an accomplished duellist, or destined to be so, for the aspen wand is one of those particularly suited to martial magic.  
> <sub>— *ditto*</sub>

No doubt there's more; I'll keep researching and updating this answer as I go.