Scott, IMO you should always allow all prospective tenants to have a rental application to fill out & return. Most applications won't come back completely filled out or they won't pay the application fee. Once you have the written application, I'm sure you could find a reason other than the # of occupants to have them disqualify themselves.
This is from the MA State Sanitation Code. How many SF in the unit & how many SF in each of the bedrooms?
410.400: Minimum Square Footage
(A) Every dwelling unit shall contain at least 150 square feet of floor space for its first occupant, and at least
100 square feet of floor space for each additional occupant, the floor space to be calculated on the basis of total habitable room area.
(B) In a dwelling unit, every room occupied for sleeping purposes by one occupant shall contain at least 70 square feet of floor space; every room occupied for sleeping purposes by more than one occupant shall contain at least 50 square feet of floor space for each occupant.
(C) In a rooming unit, every room occupied for sleeping purposes by one occupant shall contain at least 80 square feet of floor space; every room occupied for sleeping purposes by more than one occupant shall contain at least 60 square feet for each occupant.
--96.237.xxx.xxx