The bed is great for my two year old even though it sits on the grown. A toddler mattress works, but no where near long enough. It fits a 49inch mattress which I'm assuming is a twin size. Haven't had a chance to measure a twin mattress, think it'll be Perfect and get a 5to8 inch so it won't sit too low to the ground. Gave it a 4star had a few scratches on certain sides of the bed other then that came quickly and easy to put together.. My two year old loves it!!!