You either choose the tubing that you want and buy the fittings to match or choose the fittings and buy the tubing that matches them.
It's usually stated in internal diameter and outer diameter. For example, 16/11mm tubing is 16mm outer diameter and 11mm internal diameter. Sometimes you might see it in imperial measurements like 7/16" ID (internal diameter) and 5/8" OD (outer diameter) which is the same 16/11mm.