Also depends on what you're trying to do.
If you're just looking for survival or a head start for next year, you could probably get away with as small as a cubic foot or so per rhizome, maybe even a bit smaller. Hops are stubborn little bastards, they'd survive (and probably literally fill that cubic foot with roots). They wouldn't be happy, though, and you probably wouldn't get much of a harvest.
If you're looking for yield, you'll wanna go bigger. I got two or three dried ounces out of my first-year potted Centennial last year in a pot that's about 10 or 12 gallons, but, like St. Pug said, growing conditions also factor in -- I get a lot of sun, and have 20+ vertical feet for it to climb.