Of the top of my head I don't, but I used to use a standard ikea double futon mattress which fitted fine. 1100mm wide rings a bell. Plenty of room for two, spent many weekend with two in it and I'm 6 foot and no stickman. It is insulated, I didn't put it in, looks like loft insulation behind 6mm ply then carpeted. Not registered as camper, but Adrian flux insured it as a camper, with photo proof of unit, cooker, and bed.