Just remember...smaller doesn't mean warmer because ventilation is most important, and ventilation means it's not going to 'hold heat'.
Larger is better because the ventilation can be up high and easier to reduce drafts directly on chooks.
Those windows were just cleated in from the inside. I put hinges on the top, made a latch out of 3/8 plywood(that's my daughter in the last pic cutting the mortise for the latch), made a pole to operate the latches and used some eyebolts and ropes to hold them open.