there's not really a specific exact place to measure on a speaker to get the diameter. I will say that I work for a speaker manufacturer (MTX) and have on my desk right now a 4" speaker and a 5.25" speaker...
the 4" one measures 4.7" from screw-hole to screw-hole diagonally and the 5.25" one measures 5.5" from screw-hole to screw-hole.

hope that helps.

If you're completely lost, just take the speaker out and bring it with you to a store like circuit city or whatever and hold it up to the speakers they sell, and see which one lines up with it.