Yeah, squeeze as many 6mm figures on a base as you can :-)

I agrree that 60x30mm looks very good (and so does even larger bases like 80x30) but I think 40x20/30 gives you a lot of flexiblilty and more units (pretty much what you said!).  I think it would depend on the rules, some like DB*, ADLG are single element units while FOG and Hail Caesar are mulit-element units.