How do individuals or small groups connected by a radio or similar connection, one or both having minimal tools agree on a system of measurement. Assume no communication problems. Do not assume shared culture or race (no assuming their feet or screwdrivers are the same length) precision should be at least good enough for trade and navigation.
For clarity, don't bother mentioning metrics or physical constants unless you can call a highschool graduate and tell him how to base a ruler on that over the phone.
Assume transportation difficulties, ship wrecks or distance or something. (the store is out of reach) Don't assume the individuals communication tools are things they built instead of purchased or found. Assume they are on the same planet, not necessarily Earth. Don't assume either knows where they are or has fast transportation and a way to trace the radio or whatever.
Don't assume either has a ruler, they might each be able to guess near the length of their radio in their favorite set of units.
I'm asking about what is possible. Does it maybe appear too story based because I'm trying to ask broadly to draw multiple usable answers so solutions will be of benefit to many?
Based on this being my first question, being on hold, and recieveing brilliant answers and comments that made it clear I had asked badly; rather than try to fix this I'm going to close and try again. I thank everyone for their responses.