It's a matter of preference. If you plan on filling or stuffing the fattie, I'd definitely use 2 lbs of meat. If you're not stuffing, you don't even need a bag, just throw the roll in the smoker. And yes, a 1 gallon bag is perfect for rolling out 2 lbs of meat. I make enough fatties so that I go to the dollar store or Aldis and buy the cheapest bags you can find rather than using top name brands that are expensive and are only going to hold the meat for a matter of minutes, then get cut up.
What kind are you planning on making? I'm always lookin' for new fattie ideas.