I wouldn't be too concerned about warmth in the Winter if there is a pair or more of bunnies. They will snuggle up together and in my experience manage to keep warm. With a cat-flap and a closed door, there shouldn't be any drafts. A hutch inside or a wooden box as a hidey-place would be welcome inside the shed. I don't have either of these, but they do have a cardboard box filled with hay, which they use instead. This needs to be replaced every so often as they destroy it. I think it's a case of knowing your bunnies. Mine would likely nibble anything wooden and sometimes they will use the hay inside to wee and poo, so I prefer something I can easily replace. Also you need to be sure that if using cardboard they don't eat too much of it.
8 x 8 would be a nice roomy size for them