We were at YC this past June. The daybed in our room was about 6 ft. long, but it was pretty wide. I don't know if they fold out. We never did try.
I took a nap on it one day. A good 2 hour nap and I was very comfortable (I'm 5'10"). DS (12) was comfortable sleeping on it, but DS (16) was less so (he's 6'2").
I would say it would be comfortable for anyone less than 6 ft tall. The cushions are very firm...much like a mattress...you don't feel like you're "sleeping on the couch".