DO NOT MEASURE RIM OR HUBCAP SIZE - A tape measure will give you the WRONG size! It will measure a little over an inch larger than the actual wheel size.

Rim size is the Diameter of the BEAD of the TIRE, which cannot be measured without removing the tire from the rim.

All tires have a "Size Series" printed on them that contains the wheel / rim size. This series of numbers usually begins with a P (for Passenger) or LT (for Light Truck)

Examples:

P185/70/R15. LT225/80/R16.

In this designation the two digit number following the R denotes the Rim / wheel diameter is 15 inches. The R itself actually means it is a Radial tire.

The diagram below, from the National Highway Traffic Safety Administration,
shows the rest of the information printed on tires. This website also includes much more tire safety information, and is worth a visit.