depends on the size after you convert them.
and the size depends on compression, quality, bitrate, etc, etc...
the size of the AVI right now won't tell you much since that will change as you encode them to DVD, so you'll just have to go ahead and give it a try. nero vision express has a pretty accurate compression bar that tells you what will fit as you add video files to be converted.
be warned that encoding AVI files to DVD takes a LONG time depending on your comptuers specs.
creating a 6gb dvd on my P4 2Ghz, 512MB ram system takes about 5 hours or so but YMMV as always.