How high across the back or wherever the roosts will be? Normally, about 24 to 36 inches and best to make them all one level if there is more than one, otherwise they will all try to crowd onto the highest one.
To help with cleaning, make the roost so it pivots up and out of the way (which means you want it no higher than halfway to the ceiling.....it has to have room to pivot to). You do want it higher than the nest boxes or they may try to roost in those. They will go for high ground......wherever.....whatever that is.
If they have trouble getting up to the roost or learning to get up to it, you can stage some intermediate platforms.......training wheels...... (stacked cement blocks, firewood, whatever) until they learn where it is and can fly up to it on their own. Once they start using the roosts, you can clean out the training wheels.