It depends what the wand is for. If for a certain type of spell category, such as wisdom spells, use a tree branch associated with wisdom, such as willow.
Also, make sure you feel ''connected'' to that particular piece of branch, and thank the tree's spirit or it may ''reject'' you.
Wands should be about 13 inches long, but remember, the body has a natural wand. -The index finger! Which is especially good for energy channeling.
Here is a link that may help for you to decide on what type of branch you would like:- http://www.witch-crafts.co.uk/page_1194863.html
Hope that helps!