DH is 6'2" and about 320lbs and had no problems. I'm about a size 18/20 and I as also fine other than trying to flop myself into the tube and scoot down to start. I felt self conscious, but I'm sure no one even cared!
There is one inner tube slide and one without a tube that is completely enclosed/dark. I'm short (5'2") and I tried sitting up on the slide and bumped my head a little bit - so most adults will need to lie down.