How Do I Burn long DVDs in iDVD?

I have had success burning DVDs in iDVD so far, however I now have two movies that are rather long and I cannot seem to decrease the size of the file to fit on one blank DVD? I have tried decreasing the Encoding Quality to the lowest level but that still does not make the movie small enough to fit onto a DVD. Does anyone know how I can fit longer movies onto single 4 GB DVD's? Perhaps either by changing something else in the Project Info, or by downloading another application or encoding software? Thanks!

Roxio's Popcorn or Toast can both re-encode up to a 9GB video (non-encrypted) to fit on a single layer 4.7GB disk.