For extract and partial mashes, you can get away with a 5 gallon. Once your go all grain (and you will), you will need the 8+ gallon pot. I am now limited to 3 gallon all grain batches because I went cheap and got the 5 gallon pot.
Bill from Pa
On Deck: Irish Stout,
Primary: Pumpkin Ale
Completed: Red, Wit and Blue, Irish Stout, Red Ale, German Style Amber Lager, All Grain Brews: Irish Red Ale, American Stout, Honey Weizen