The basing is entirely up to you and your opponents.
Decide on a 'standard' sized unit. Use that frontage. 50% bigger becomes a 'large' unit and 50% smaller becomes a 'small' unit. That being said a few mm either way is
NOT an issue.
e.g. Don has his Romans based up in 24 figure units, 3 deep on 25mm frontage (unit frontage of 200mm), James has his Romans based in 16 figure units 2 deep on 20mm frontage (160mm unit frontage), BOTH units are considered 'standard' units.Don of course has the option potentially to make a few more units out of his extra figures (assuming he has them based in such a way as to allow removing the third rank), or perhaps deploy them two ranks deep giving a unit frontage of 300mm, which would then make his cohorts LARGE units.
The tightly regulated basing schemes of other ancient rules are done away with.
This becomes extremely liberating. But also confusing to those used to more tournament oriented rules with stricter basing.
My 15mm armies are based on 40mm wide stands, two stands to a unit. So 80mm frontage per unit. 12 HI in two ranks or 6 skirmishers scattered about as a small unit occupying the same front.
So individual figure frontage DOES NOT MATTER!
Actually unit frontage doesn't matter either
just as long as everyone is close to being the same.
I hope that helps.
James