I use kings, from an old waterbed we used to have. They're stitched together along the bottom, so I find it easier to make up the bed. (Easier is a relative term.) We keep an imitation feather bed on top of our Casita
cushions. I expect a fitted queen sheet would work just fine to cover it all... I also expect a full would do the same. (The bed is NOT a standard size; besides being a little shorter and a little bit narrower, remember the two corners along the back of the trailer are quite rounded.)
Hope this bit of rambling helps!