15X15 Room . If you want to go bigger i would do something like below. Browse 15x16 bedroom and on houzz. Bedroom Design 15 X 15 Home Architec Ideas from www.1001homedesign.com One gallon can of paint will cover up to 400 square feet, which is enough to cover a small room, like a bathroom. How big is a 15x15 room? This means that the room is 15x15 tiles.