Your initial box measurements sound good though I also like the idea of doing some boxes bigger and some smaller if you have giants and bantams.
How many chickens do you have (or plan on having?) 8 boxes is a lot so another option is going down to 6 and having all very wide boxes, but that depends on how large a flock you're aiming for.
How many chickens do you have (or plan on having?) 8 boxes is a lot so another option is going down to 6 and having all very wide boxes, but that depends on how large a flock you're aiming for.