Answer Wiki. THE AVERAGE MASTER BEDROOM SIZE? 200 to 250 sq ft or if you want an idea of the dimensions 14x15, 15x15 or 15x16 That is just the average size. If you want to compete in todays market you need to go with about 350 square feet which would be around 20x17, 19x18, 20x18, 20x19, etc.