I use one size smaller id than the barb size for my gas lines, so as the barbs are 5/16" od the gas lines are 1/4" id.
This ensures a tight seal even without a clamp (which I still do use)...
I unified all my tubing, so I am using 3/16 ID x 3/8 OD for both my beerline and gas line. It doesn't matter the size for the gas line with the lengths that we use. The sizes that have been used, and are sold as gas line are just the convention. You can decide what size to use that fits your needs best.