If you have a measuring caliper, use that to measure the diameter directly. Sizing is metric, so if your caliper is English-only, just remember there are 25.4 mm in an inch (exact conversion).
If you have a flexible measuring tape, you can measure the circumference and divide by pi (3.14159 blah blah blah) to get the diameter.
If you don't have a flexible measuring tape, you can take a string, wrap it around, then hold it up against a ruler or rigid measuring tape.
Your calculated measurement will probably be a little off, of course, but there are only so many sizes they make - just round it.