We are using a full mattress on top that is ~12 inches tall and fits well up to the bottom of the back piece with little gap showing. We have a ~10 inch twin mattress on the trundle and don’t think you can go much bigger or it will not close.  Warning: when putting this together, the pieces are not labeled so make sure you pay attention to which side of the back and side pieces the material is on to avoid having to re-do the assembly (as first round the darker lining was on the outside versus the dark grey material). NOTE: the back piece and side pieces (in front and on back of bed are NOT lined on all sides of them. As such, you would NOT want this in the middle of a room, must be against a wall.