a 20 oz tank will last you all day for the most part. Unless of course your like me and shoot 2-4 cases a day
I personally always prefered the chromolly tanks which are the round ones, instead of the flat bottom cylinders. the 16 and 20 oz ones like those were nice. If you plan on playing at a field then a 20 oz will be just fine. If playing in the woods id say get 2 just incase, which a 20 oz is around 20-25 shipped so not very costly.