The Narrow Small Bathroom Layout depicts a bathroom with the smallest size of 15 square feet, which can accommodate a sink and a toilet. A shower requires at least 30-36 square feet to be installed. A full bath with a shower and a bathtub will need 40 square feet. A small bathroom is typically 35-40 square feet in size. There is no certain regulation defining a minimum bathroom size, but the sanitaryware and your ability to move in the space necessitate a certain amount of space. According to the Narrow Small Bathroom Layout, one of the most common narrow bathroom dimensions is 1m wide x 2m long.