I've never had any problem with exact dimensions on a nest box. Growing up near Ft. Hood, we were always buying surplus wooden ammo boxes for 25-50 cents a piece. The army recycles most of them now days. We used a variety of mortar, artillery, and rocket boxes for all kinds of projects including owl boxes. They were different sizes & shapes. We built nest boxes generally using a bow saw to make a couple of parallel cuts about 3 inches apart and about 3 inches deep then knocked the middle out with a hammer to create an entrance. Screech owls, squirrels, ringtails, and other things used the boxes for nesting and shelter. The most used owl box we had at that time was only about 20 feet from the front door of our house & about 8 feet high sitting flat on a horizontal limb of a big pecan tree. This house was used constantly for nesting and as a day roost out of the nesting season. It seemed there was always an owl in that box. I'm sure there are some sort of min/max dimensions out there but I haven't found them yet. The owls I've had around were not picky, just glad to have a cavity available.The quickest occupancy I ever had of a box was about four hours. A friend called & said he had a "baby" owl sitting in a tree in his yard for the last several days. It was not a time of year babies would have been around so I told him it was a screech owl, and I had an extra nest box if he wanted it. He did so we put it up about four that afternoon. At about eight that evening he called excitedly to tell me that he was watching the owl sit in the doorway of the box while he and his kids watched from their patio.There are always more owls that trees with suitable nest cavities so these owls respond pretty quickly at times. The biggest problem with the boxes is that squirrels find them attractive as well. When a squirrel moves into a box, I generally take action to remove the squirrel from the area. A live trap, a little corn, and a pellet rifle at very short range work well in town.Screech owls are small and won't leave the volume of white wash as barn owls in south Texas tower stands. I have never had a problem with screech owls making a mess. Most of the time you will never know they are there. I have a little homemade pond with waterfall and the owls use it for water and bathing. Just something else to watch them do in the yard.