There is no real relationship between the length of a movie .. and the size of a resulting dvd, as all files can be compressed to get them on the disc.
If you import to PC and edit them as AVI, I would recommend you use VSO ConvertXtoDVD to convert.
With the quality you are using, you could produce a dvd with more than one title for each DVD .. and have around 2hrs 20min of run time without appreciable quality drop.
Check out the forum stuff here ..
http://forums.vso-sof...ides-manuals-f23.html