I'm guessing that you shot it on film - one of the nice things about digital of course is the instant feedback. You can make adjustments because you know what's working and what isn't. I guess I'd stick with the flash to freeze motion and a telephoto like maybe a 200 to compress distance (make the rain drops look closer together). Probably will take some experimenting.

16x20 out of a D100 RAW file? No problemo! I've gone bigger with jpegs from my Fuji S2.