I have chickens that roost in trees which in all honesty looks rediculous, but if they have the space they can go pretty high until they get too fat for their wings to support them...lol
I'd go with no higher than 12" in a 32" house, they should be able to just pretty much jump on it without banging their heads there ....mine range from 12" to 48" high, but I have an 8 foot ceiling so there's plenty of room for flying up......