Twin and full mattresses are both suitable mattress sizes for a small apartment bedroom or studio apartment. A twin mattress measures 38 x 75 inches and can fit comfortably in rooms as small as 7 x 10 feet. A full mattress is slightly wider, measuring 54 x 75 inches, and can fit comfortably in rooms as small as 10 x 10 feet. Thanks to the added width over a twin, it's roomier for solo sleepers. A twin XL, a variation on the standard twin, is commonly used in dorm rooms. As it's only 38 inches wide, it easily fits in small spaces. But since it's 5 inches longer than twin and full beds, it's ideal for tall people.