There isn't a fixed base size for BKC - which is good if you have an existing army, but not very helpful if you are starting from scratch
The sizes SCAdian gives above seem to be ones used by many people in 10mm. 40x20 would work, but would give quite close spacing to your figures (or you could get away with 3 figs on a base).
These are photos of my British Para battlegroup
http://www.kerynne.com/games/BritishParas.html the infantry are on 50x30mm, the guns and HQs on 40x40, jeeps on 40x20mm. I tend to use 30x50 for tanks.