No I don't have them yet but it's probably not a bad idea to start thinking about this now. I've pretty much settled on 20mm x 40mm for the bases, I'm sure there will be the odd tank or arty piece that needs a bigger base but the benefit of cutting your own is that you can make the exception where needed. I'll still try to keep the bases fairly uniform; eg bigger tanks might go onto a 25mm x 40mm and large arty pieces may even push it out to 30mm x 40mm.
The one complicating factor is that I really want to base my miniatures so that I can use them for other rules. One of the great strengths of any hostorical game is that the miniatures are still relevant regardless of the ruleset, basing pending of course. The areas where I can see things getting tricky is is with support weapons: MMG, HMG, mortars and such. I'm thinking that I can base these individually on 20mm square bases and simply put two side by side for Blitzkrieg Commander (BKC). To make this easier I've bought 200 adhesive magnets:
They are the same size at the bases I'll be using and are 0.6mm thick. They'll help with storage and will also allow me to make some unit sized movement trays out of paper steel (or something similar). That's the plan anyway. They will also help with some of the Warmaster minis which are just a pain to move as a unit (yes I'm looking right at you Carrion).
These are coming from Adelade so they should be here in the next couple days.