Height of roosts can depend on size of coop, they need room to fly/jump down without crashing into something, or be provided functional ramps.
Here's some tips on heights:
https://www.backyardchickens.com/articles/coop-stack-up-how-high-stuff-works-well.73427/
Might also depend on how you plan to manage manure, I use poop boards.
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Question: @aart How high up are your poop boards? I am thinking of raising mine to make sure there is plenty of room underneath for my girls to wander. However, they only really sleep in their coop, so it might be irrelevant.