So it is for incubating the eggs, not keeping the chicks in with the hen until she is ready to wean them. Makes a difference. So you need enough room for a nest (12" x 12" maybe), room for a feeder and waterer (what are you using), and a bit of room to go poop. She should know by instinct to not poop in her nest but that doesn't carry through to anything else. You will need to be able to access it for cleaning and to change food/water without too much pain and aggravation so don't make it too big.