Sizes for sanitary ss tubing, like other tubing, is based on the outside diameter. ID varies by wall thickness – gauge. A typical wall thickness for sizes under 2.5 inch is 0.062 inches. mgp has provided you some excellent sources of the detailed information.
Sanitary ss tube is the standard (vs. pipe) in Pharmaceutical, semiconductor, and food for a number of reasons. It is available with polished (and electropolished) interior surfaces which is critical to maintaining sanitary conditions. It is fabricated with very close tolerances which make it ideal to produce very high quality, repeatable welds using automatic oribital welding machines. It is made with very high quality stainless alloys, which again contribute to making high quality, corrosion resistant welds. Because tubing is superior in these aspects, the components (valves, connections, instruments, etc) that have been developed over the years for sanitary systems are all built for tubing systems.
For semiconductor applications, polypropylene and PVDF tubing/pipe have widespread application, were metal ions are an issue.