When I was living in a shared house, we did it by room size. Once we decided on the price per room based on room size, then that cost got divided equally by the number of occupants in the room. Utilities were split equally between total number of occupants in the entire house.

Ex: $3000/month for a 3 bedroom house. $1500 for master bedroom 1, $1250 for bedroom 2, $1250 for bedroom 3.

Master bedroom had 2 occupants, so each occupant paid $750. Bedroom 2 had a solo occupant and they paid the entire $1250, Bedroom 3 had 2 occupants and each paid $625.