In my two 4x8 coops, my three nests are two feet high; each is 15-x15-x18-inches high. I have a 2'x3'x9"-high black plastic cement mixing box above the nests and immediately below the roosting 2x4's to catch the poo. I have about three inches of equine pellets in the plastic boxes that I mix with the poo and diatomaceous earth. I might have to dump it in a month or two; meanwhile there's no smell or bother other than adding the d.e. and mixing with a small hand shovel.
They do need wingroom to be able to get up, though.
Our coop is built in 2 sides, an enclosed side where they roost and nest and sunroom side where they eat and drink. We originally built our nestboxes up on a shelf about 15 inches high, and then there was a set of roosts up about 3 feet. What we didn't realize until we saw them using it was that there wasn't enough space between the edge of the shelf and the roost - the angle was too steep. Sometimes they'd make it and sometimes they wouldn't. So we build on an enclosed 'windowbox' extension to house the nests to reclaim all that floor space, and installed a lower roost where the shelf had been. Now they jump onto the low roost and then up to the higher roost with no problems - plus we can get to the eggs from the outside, which is easier for us too.