You want the size of the speaker itself. Sounds like it is probably a 6" speaker, but it might be an old 6.5" car speaker somebody put up. Measure the distance across mounting holes, then check some of the online places like Parts Express etc. to see if you can find one with the same dimensions. Or, take it out and to a friend or your local Radio Shack dealer.
You might be ahead just trashing the old speaker (enclosure and all) and picking up a new outdoor speaker to mount in its place. Do you know the wiring to it is good, and where it comes out?
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley