The nest box should be large enough for the mother to sit in comfortable as well as to turn around in it. 1.5-2 times the size of the rabbit is a good guideline. The box should be provided 28 days after mating the rabbits. Providing it too early will cause the mother to just poo and pee in it instead of making a nest. Around 28 days, the mother will begin collecting hay and pulling fur to create her nest in the box.