Where is the proper location to measure the tube size?
Look at the tire.  It will have the size embossed on the sidewall -- either inches diameter &amp; width (like 26x1.5) or metric sizes (like 700C-32).  Use a tube that says it's for that size.

Tubes generally are made to fit a range of sizes.  Eg, a single tube may fit both 27" and 700C tires, and 1.25-1.5 inches or 32-38 mm wide.