I do three gallon boils in a 16 quart pot and you do have to watch it like a hawk when it first comes to a boil. I do wish I had bought the 24 quart pot for the same three gallon boil. (Had the choice of the 16 or the 24 at Sam's Club, commercial quality heavy gauge aluminum pots, the 16 was about $25 and the 24 was about $32, great buys I thought).
One thing to bear in mind is the capacity of your stove to boil large amounts of liquid, even with a gallon of preheated liquid from steeping specialty grains it takes three gallons of wort at least half an hour to come to a full boil on my electric range.