Larger the better. 10# an 20# only cost slightly more than a 5# to fill. A worker told me that the most expensive part is hooking the tank up and the co2 is cheap. Also, you can buy cheap tanks online at Craigslist, but they must have the ID tags. If they don't, places won't swap or fill. I use a 10 and fill every 3 months or so and a 5 I fill if, I have "extra" money, to force carb separately.