Reply to befuddled: There are large, and large, and larger still! I would suggest a minimum width of about 35-40cm (14-16in) across and at least 35cm (14in) tall. No problem choosing something larger. The bigger the pot, the more compost it holds, and greater water reserves. Make sure the pot has good drainage holes in the base, or drill some more. Place a layer of crocks or gravel in the base of the pot (about 2.5cm (1in) deep to make sure excess water drains away and drainage holes don't get plugged. Standing pots on small feet also ensure drainage holes remain clear and free-draining.
Heavier materials provide greater stability than plastic pots, too.