I agree with DDawn. That's about what I do, put hardware cloth between the rafters on two sides and left a cut-out on the other two sides, also covered in hardware cloth. Just have an overhang to keep the rain out.
On roost height, I suggest as low as you can get it yet make sure it is higher than anything else you don't want them to roost on. Mine are about 4 feet, by the way, and I have similar sized chickens as yours.
Chickens can hurt themselves jumping down, the bigger the chicken, the more the risk. That does not mean they hurt themselves very often. When mine hop down, they spread their wings and flap to slow their descent. It is not that you absolutely have to have them low. I just consider it good practice.
Another issue is that the higher they are, the more horizontal clear room they need to fly down. Chickens, especially big chickens, are not real graceful flyers. Mine have launched from those 4' high roosts, flown forward six feet, turned a 90 degree, flown out the door, and landed in the run, so they can fly, but they are just not graceful doing it. If they don't have plenty of room, they can hit walls, nest boxes, feeders, waterers, ot anything else. Again, just good practice more than anything else to try to keep them reasonably low.
I would not worry about the window being open and a breeze on them in the summer, but keep that window closed and the roost below the vents up at the top. They will do fine.
Mine will fly up four feet to get to the roost. My nest boxes are along a side wall and about three feet back from the roosts. Most of mine normally hop up to the top of the nest boxes, then hop over to the roosts, but some do fly straight up. With the roosts five feet, I would give them an intermediate step 2' to 3' high. In the morning, I'd be surprised to see them use an intermediate step down. Mine just hop/flap/fly straight to the ground.
Good luck!!!
