I needed a tank that would fit in the fridge, so I went with a 20 cu ft tank, which is about the same size as a 5 lb CO2 tank. Just like with CO2, it's much more cost effective to go larger if you can accommodate it. When I got my tank I took it to General Air to fill and they told me they won't fill tanks that small and offered to sell me a larger one, I think 50 cu ft IIRC, for around $100 + the gas. I declined and headed over to Airgas. They couldn't fill mine on the spot, but they had full 75/25 beer mix tanks in stock that were the same size as mine, so I swapped mine away for a full one for just the cost of the gas, which was ~$38 out the door.