As a quick guess ~ 0.020 - 0.050" smaller than the butt od in the rod specs should get you in the ballpark.
You could wrap a thread around the rod a couple of times and then mark the thread at one location. Unwrap the thread, and measure the length. To get the diameter divide the length by (pi * # of turns).
If you need better accuracy spring for a micrometer or calipers.