A standard single bed is 3 feet wide and 6 feet 3 inches long (91 x 190). This is the most common bed size for children, kids beds and bunk beds. What size is a Small Double bed? 122 x 190 cm / 48 x 75 / 4' x 6'3. A small double bed refers to a 4 feet wide, 6 feet 3 inches long mattress (122 x 190 cm).