It looks like a good size to me. I recommend using a flat piece of wood or a thick branch for a roost though instead of a dowel. I've read that dowels are hard for chickens to grip and can cause some feet problems. I use a 2x4 piece of lumber and it works great. Some people use 2x2's, but I wouldn't use those for large fowl myself. My chickens can't seem to grip well on anything that's thinner than the wide side of a 2x4.
As for storm windows, that depends on the amount of bad weather you get. I think it would be a good idea to have something to protect them from rain coming in, even if it's just a tarp over top during rainy days. I have a lot of bad weather here, so I made windows that can be closed during inclement weather. Nice find on the playhouse! It's very cute.