For questions on local building code it's always best to check with the town's Building Dept..you can reach yours at 617-730-2100. If you'd like to discuss how adding a bath can add value to your home please contact me.
You should probably check with the Town's building inspector. When you apply for the building permit, he should be able to answer all those questions. If you do not get a permit, it doesn't really matter what size the door is, it won't be legal. Any future buyer will ask to see the permits and if there is none, it could pose a problem down the line.