I don't keep a container in the coop for dust bathing. I keep it out in the run. I'm using the bottom of our old guinea pig cage. It's about 3'x2', I'd guess. It's deeper and larger than a kitty litter box. It's full of construction sand and cold wood ash. My white chickens are no longer white! LOL And, now that part of the run is covered, I don't have to run out and cover the bath tub when it rains to keep the contents dry.