Not sure what kind of file you made, but if you want the DVD to play in a DVD player, the video has to be MPEG2. The only thing that controls filesize in MPEG2 is the bitrate. 1.5-hours of video should fit using a bitrate of 8Mbps.
You control the bitrate in Vegas by using the “DVD-A” preset in the MPEG compression options.
You can also let DVDA make the decisions for you by simply opening your AVI file in DVDA. DVDA will select the encoding parameters for you to make the video fit on a DVD.
Steve Mann
MannMade Digital Video
http://www.mmdv.com