Rocha Fernandes Furniture, 2018-01-07 02:44:05. The twin mattress size can be a bit deceiving. At 39 inches wide and 75 inches long, it is the smallest standard mattress size available. You would have to go to a crib mattress size to find anything smaller. But for the twin mattress size, there are still a number of uses for it. Follow along for much more on this dynamic mattress size! Twin Mattress Size: Uses and Information It could be a tight fit for most adults. At 39" x 75", it is too short for those taller than 55", according to The Better Sleep Council - though adults in tight living quarters can make the jump to a twin XL (39" x 80").