It is smaller than a twin (bit shorter, not as wide). It fit my large 9 year-old (more like 12-year-old sized) fine, but it would not fit a taller/larger adult (probably not over 5'4).
It fit my 5'7" son just fine last summer. He had several more inches of space. It's a standard twin length mattress at 72" long. This kid sleeps on a twin at home just fine as well.