The physical dimensions are:
Small Base: 1 1/4" x 1"
Medium Base: 2" x 1 1/4"
Large Base: 2 1/2" x 2"
Smilin' Dave's list is pretty dead on. The game designers themselves have, however, mentioned on the Battlefront (
FoW) website that those base sizes are but guidelines, the key thing just being that you're consistent through your army. The group I (infrequently) game with all started off with Command Decision and our armies were orignally based using that ruleset's suggestions. Despite being slightly smaller than the
FoW system, it hasn't adversely affected gameplay in the least (same for vice versa, that is, using
FoW basing in CD3/4). An advantage of the slightly larger sizes for
FoW is that they do allow more dioramic basing. Enjoy!
Ps. You can also vary the number of figs on each base for variety's sake and/or to stretch out your dollars. Same stats, just a base with less figs = battle hardened vets, while one with more figs = green repple depples. Whatever yer whim may be.