Twin mattress dimensions, what size is right for you?. A twin mattress is the smallest width and length with the actual dimensions 39 x 75 inches or centimeters 99.1 x 190.5. The total area of the mattress was 2925 square inches or square centimetre 18.878. This is the best choice for one person in a small bedroom. Located in the middle of the room twin beds will allow about 40 inches of space from both sides and 45 inches from the long side of the bed for bedroom bedroom wall 10 x 10 feet. Will perfectly accommodate two children and up to 27 inches in the closet. Placed at the corner of twin beds opening larger areas and larger dimensions drawer may be easily incorporated in the design of the room if the room layout matched.