I have five nests raised a few inches off the floor about 12" X 12" X12" and 12" X 12" X 18" wide, three on top of two. The top middle one is the favorite of those 5, and it is the smallest. I also have two large plastic bins on the floor; one is an upturned kitty litter box top. One of these is the favorite of all. The only two they haven't used are the two lower larger nests. In my other coop, there are two, one a 12" cube cardboard box on the floor, which is their favorite. The other is about the same size but about 18" off the floor. The broody uses the top of the higher one for sleeping with her chicks, but no one lays in it. Yet it was the favorite nest before I put the cardboard box in. I checked it out for smell and lice and changed the bedding and... they still use the cardboard one.
I won't bother to raise them off the floor again unless I need the floor space.