I've got a round face too, and getting short on the sides, longer on top helped. It really squares you out. For round faces, you have to have short hair on the sides, or else you're just adding additional volume and making your face rounder. You also should have relatively short hair on top, and comb it up, not down over your forehead. Combing it down reduces the length of your face, making it seem wider, whereas combing it up makes your face appear longer! Sorry to ramble, I just love chatting up the ladies who cut my hair.

As far as what I've done, I've gotten a 3 on the sides with a 5 on top, a 4 on the sides with a 6 on the top, a 4 on the sides with an 8 on top... really, just choose something between 2 and 5 for the sides, and between 5 and 8 for the top, and you can't go wrong. The variation among the different lengths is negligible (or at least, it was for me!)
Edited for photographic evidence, I believe this was either a 3 and a 5 or a 3 and a 6:

sorry for the poor quality!