If youhave a pump the bigger the better, it takes up way less space to have 1 6 then 2 3'ers and you dont have to lift except when empty to clean. If making from fruit then I like to have all of the above and then some. This way you can keep racking down and never need topping up. I have all sizes. 6.5, 6, 5, 3, 1, 3 liter, magnum, 750ml, and 375ml and the bungs to match!