Twin

A twin mattress measures 38 inches wide by 75 inches long, the smallest of the six US standard sizes. It is usually the first big bed a child moves to after a crib. Sellers also label it single, and both names mean the same size.

Those 75 inches are the real constraint. A 5 foot 10 sleeper has roughly 5 inches of clearance at head and foot combined. Anyone close to 6 feet ends up at the very edge once a pillow takes its share. Width sets the other limit: 38 inches lets one adult lie flat with arms at the sides, and not much more than that.

Floor space is where a twin works best. Two of them fit across a room that would hold one king. That is why bunk beds, trundles and daybeds are built to this footprint and take no other mattress. Frames are light enough for one person to carry, and the mattress turns a tight stair corner that stops a queen.

Giving up room to gain room is the whole bargain. A twin costs the least of any size to buy and to dress, since bedding in this size is cheap and stocked everywhere. What you cannot do is share it or grow into it. A tall teenager will meet the footboard, and the answer there is Twin XL, because a longer twin does not exist.