Full size mattress dimensions are 54″ x 75″and are a great fit for teenagers or children. In order to fit a Full size mattress while still enjoying optimal living space, a bedroom should measure at least 9 x 9.6 square feet in size. However, the average room dimensions for a Full size mattress is 10 x 12 square feet. The Full size mattress, also known as a Double bed, is not actually double the size of a Single Bed, but only 15 inches wider and no longer. For co-sleeping adults, the Full size mattress will offer only 27 inches of personal space, making the Double Bed better for singles!