Where is the proper location to measure the tube size? I need to replace a tube in my bike. I know there are many tire sizes. I bought 26', assuming that my bike was that size, and realized when I got home that I didn't even know where to measure from. What is the correct location to measure in order to get the correct size of tubes?
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.