The size of mattress needed will greatly depend on the person(s) who will use it. If you are single, you might like a twin or full size mattress. For a couple you’ll want at least a queen size mattress, but more likely a king size.
Keep in mind, the size of your room and the amount of space allocated for the bed. A king measures 76” x 80” and can be too large for some rooms. If you are unsure, it’s wise to take a measurement beforehand.