The way we did it was to evenly split the common spaces among the renters, while accounting for room size.

individual_rent = total_rent * (room_size + common_share)/house_size)

Utilities are split evenly among roommates.

It works, although I probably would have prefered to use something like the system here were I to do it over. Things like "has a master bathroom so you don't have to share with everyone else" tend to be worth more per square foot than regular bedroom space.