Take a fabric tape measure and wrap it around your shaft (erect, and about the middle of the shaft). Take the measurement where the end of the tape meets the tape.
If you can't get a fabric tape measure, you can use a strip of paper or some (non-stretchy) string. Wrap it around, mark where it meets itself, then straighten it out and measure with a ruler.
Length doesn't matter for condoms, because they are made long and you only unroll them as far as needed. The big ones are only a little longer than the small ones. But girth affects tightness.