What soft tubing size should I use?

This will depend on your particular use case. 3/8x1/2” is technically more flow-restrictive than some of the larger ID options like 7/16” or 1/2“ ID tubing, but takes less room and draws less attention than the larger sizes. Thicker walled tubing like 3/8x5/8” is less prone to kinking around tight bends, but takes up more space. If you want the tubing front and center, minimal flow restriction restriction, or just want to show off the fluid inside, 1/2x3/4” is a good choice. 3/8x5/8” is a solid middle ground choice with a balance of pros and cons that make it the most common go-to choice.