We played a lot of KoW v1 with 10mm figures, I think we used 80mm by 60mm for a normal infantry regiment (basically 2 warmaster units side by side), along with standard measurements in inches, it worked well.
It might be worth adding a bit more frontage to cavalry, large infantry and '25mm based' infantry, to represent their bigger frontage in the 28mm game, 120mm or 125mm frontage compared to 100mm for infantry. It makes them a bit more unwieldy, and allows more attackers against one. Not a game breaker, but does add a bit more variation to the units than just the stat line