We buy king singles fr our kids. An adult can easily fit if needed. You could start out with a trundle, as it is lower to the ground.

We just used a kids small, fairly flat pillow. I do find she bangs into the wall a lt, so I have a long sausage pillow along the wall to stop her slamming into it. Also have a bed rail to stop her falling out the oter side as she flings herself around in her sleep too.