First question is......... HOW LARGE IS YOUR COOP GOING TO BE???
All things will be relative to overall size. Now here are some guideline answers to your questions.
Roost ......... 12 inches away from wall is sufficient. Height. ,,,,,,,,,,,,,,, 3 to 4 feet off the floor is good. Headroom...... this is where it gets technical. All depending on total height of your coop. Ventilation needs to be at top of coop. It needs to be such that there is plenty of ventilation, but no direct drafts blowing on chickens.
Nesting boxes are best if somewhere about 12 inches off the floor. Reason for elevation, is to keep chickens from scratching litter into nesting boxes. Size may be 12 x 12 x 12 to 16 x 16 x 16.. All depending on size of your coop. You only need about 1 nest per 4 or 5 chickens. They will usually share a few and not use some of the rest. If you intend to keep 10 chickens, 3 nests will do.
WISHING YOU BEST and