What size does a nest box for pigeons need to be?

I am going to be keeping pigeons soon for the purpose of dog training. I have already built the hutch, but now need to construct nest boxes for extra shelter and protection (and for use after mating). What are sufficient dimensions for one nest box, and does it matter if the next box is secured above the floor of the hutch or on the floor? Any advice will help. Thanks for your time.