The AoS rules don't use bases sizes for anything, so - unless your group house rules measuring/stacking/something-else - you don't need to change to a particular size (or at all).
That said, if you are intending to play pick up games there is likely to be one opponent who disagrees with whichever decision on basing you make; this will still happen if GW do release a list of mandatory base sizes.
So far it looks like human sized models are all on 32mm bases. For example, the re-released Dryads and the Bloodreavers in the box set are on 32mm. With the slightly larger humanoids on the 40mm bases. I guess the 25mm bases would be best for smaller things like Goblins and Clanrats.
But then again, if the bases don't matter then I guess it's up to the individual. It would be nice to have some sort of guide though.
I was hoping to paint up some zombies and I think that 25mm bases would be better as they would look more like a shambling horde being packed closer together. But then again GW might re-pack them with 32mm bases and there's bound to be someone nit-picking about them. Hopefully they will update the boxes sooner rather than later.
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