I go the large 35"x35" in cream for my 65lb dog who loves to burrow. 
He fits perfectly in it when stretched out, but I think we could have gotten away with the medium too.
It came in the smallest vacuum seal pack and I was very worried about the pad fully fluffing, but it did just fine. I have since washed the cover and it unwrinkled just fine as well. It comes with a pole to run through the top of the hood to help it stand a little bit. May eventually stop using that because my dog likes to be fully covered or to just lay on top of during the day.