Minimum tank size is now 1500 gallons and a system must accomodate at least 3 bedrooms. The size of the tank/leaching field is determined by the number of bedrooms - you can have a home with 25 rooms, but if there are only 3 bedrooms, your system is a 3 bedroom one. In fact, many towns will not permit you to oversize your system.

HTH

Correct. Determined by number of bedrooms. If something looks like a bedroom they may count it as a bedroom even if you don't intend to use it as such (has a door and closet).

Ditto was Sue36 says.

You might get some slack if you want to build a 5000 sq ft 3 bedroom house with 6 bonus rooms. I doubt there is a MA specific rule on number of rooms but just look at it from a possible bedroom standpoint. Any bonus room (what we call them here) can be potentially a bedroom. You might get away with 1 on a 4 bedroom house but after that, you may have an issue.

A closet defines a bedroom for listings etc. but a room with no other purpose is a potential bedroom for the purpose of a septic system. They "round" up on bedroom count for septic systems.

Around here, these rules would be local and not state based.

A septic system in Massachusetts is regulated by local boards of health under the State Environmental Code, Title 5 (310 CMR 15) but the local requirements can be stricter than the state requirements and the DEP can be involved.

"CMR" means "Commonwealth of Massachusetts Regulation".

Just google Massachusetts 310 CMR 15 or title 5

This is something that must to be designed by an engineer so you don't need to read the code. You just need to decide how many bedrooms you want to build.

Of course, you have to prove it is not possible to connect to a city sewer system.