For me it\'s based on the game.
Warhammer has set sizes for their bases. For fantasy it\'s 20mm for most humanoids (Empire, Elves, Dark, Elves, Skaven, Dwarves). Bigger humanoids get 25mm (Orcs and Choas). Monsters and Swarms get 40MM.
I haven\'t played 40K in years but there seem to be three sizes there as well, normal, Terminators, and big bases for Demons and Dreads.
For D&D there are standard sizes as well, a really small round base for tiny, a somewhat small base for small, a base that fits in 1\" square for medium, a base that fits in a 2\" square for large, a base that fits in a 3\" square for Giant, a base that fits in a 4\" Gargantuan, a base that fits in a 5\" square for Collosall, or at least close to that.
I know Warmachine bases are bigger then 40K standard ones, but I only own a couple minis from Priateer and they are still in blisters so I don\'t know the exact sizes.
So overall if you plan on ever gaming with your minis, or if you plan on selling them to someone who does, you propbably want to stick with the proper size based on what you are painting. What I see on here alot are really nice display bases that incorporate the proper size base, either insetting it, or having it \'stand on\' the larger base.