Are you just burning the AVI file on a Data DVD? Not much you can do... You could split the file, but that may break the playback. OR maybe you can recode the video (AVI to AVI) and lower the quality.
If you convert the AVI to DVD format and it's too large for a DVD+-R, then run it through a program that will shrink it (CloneDVD, Shrink, DVDFab...).
I've been playing with AVI to DVD converters and most of them let you pick the quality and such. The lower the quality, the smaller the file.