You can go down to as small a size batch as you desire, if you mash your grain in a Brew in a Bag (BIAB); you might want to use the Brew in a Bag also as a hop bag, if you are concerned about containing hops to maximize wort recovery. But, really, a pot on a stove for a gallon or so is pretty convenient, as Denny says. Putting a colander in the pot/Foundry can help keep the bag off the bottom of the container, or simply clip it to the side of the pot to suspend it.