Smallest bin for nursing rat?

I'm rearranging my feeder plans a bit and have run into a dilemma. I need to separate my females so that they have a tub to themselves, one that's large enough to comfortably house a female and male rat, and later a female and her litter. I'm used to keeping them as pets, so while I want to use much larger tubs, they take up too much space, whereas I see several females comfortable in a concrete mixing tub. About how many quarts does a pair, or mother and litter require?