There's a lot to learn, and building that you won't have to re-do for years takes a lot of research/planning/building time. I spent about 6 months reading and planning and an entire summer building.
How high the roosts can depend on the size of coop and the available 'landing area' to avoid crash landings and ramps taking up too much space.
They don't carry tape measures :D so doesn't really matter how high the roosts are.
Put the roosts where you want them, ease of grabbing birds off...