You're definitely right that roosts should by higher than nest boxes--if it's the other way around, your chickens will probably roost in the nest boxes, which is not pleasant at all when gathering eggs.
Definitely depends on your breeds.
My roosts are 6" (Silkies and babies), 2ft (unlucky, submissive birds), 4ft (Buff Orps, Plymouth Rocks, Production Reds), and 6ft high (EEs, sex links, dominant birds). They are usually 1-3" wide. Minimum space from the wall is 1ft.
As Chicks Galore's setup...