Cyn, I've made Christmas baskets every year since forever...
and personally I seem to use the standard 'square' Size: 3 1/2 x 2 1/4 x 8 1/4, more than any other. If you are doing a lot of nuts and candies, then the 6" high one would be good too. You have to allow room at the top to tie it off with ribbon, or whatever, and if it's too much left over at the top, you can always trim it down. BTW, if you use those designer scissors (like for scrapbooking) to trim it down, it looks very pretty. It's nice to throw in one of those designer print bags, but more than 1 or 2 in a basket looks awkward.