I'd say tubing size is all about users aesthetics and how the layout of the tubes will be. If you have tight bends - thicker walled tubing is preferred.
ID is Inner Diameter
OD is Outer Diameter
always choose tubings sizes according to fittings dimensions.
i.e: 1/2" ID 3/4" OD compression fittings will need 1/2" ID 3/4" OD tubing. If its barbs that you fancy then only choose the barbs according to the ID of the tubing.
might also want to knock up martinsliquidlab site to understand just how much flow is compromised on lower dia/bore tubes.