Renee,
From experience 4 foot deep means it is hard to catch them so 3 feet is perfect. Not everyone's arms are long you know. My short Asian arm makes it difficult to grab birds like that. They just go at the back and they laugh at you when you can't reach them. These boxes prevent you from getting inside so you are supposed to just grab them from the outside. If I have it my way it would probably be 4 feet high, 3 feet deep and 4 feet long.