Slitherine

I have read that in the new list this army may have up to half the groups of "bushi and followers" upgraded to superior but, ¿may these groups be bigger than the average ones as well? (so you end up having more superior bases than average, for instance taking two superior groups of 8 bases each and two average groups of 6 bases each)

Well the revision does state that the Late Heian to Muromachi Japanese can have up to half of their "Bushi and followers" battle groups as superior. this uses the term battle group rather than bases, so I would agree with your suggestion above.