Make file size smaller?

Not really, unless you change the compression. But as MPEG-2 (the codec used for video on video DVDs) is not as space efficient as modern MPEG-4 codecs, you can only put up to 90 minutes on a 4.7GB DVD medium in acceptable quality.

How big are the files you have and what format are they in and what codec do they use?
And have you tried iDVD or Burn yet?

 

It depends on what exactly you are trying to do.

If you just want to back up the computer files, for playback on the computer; use a utility such as Handbrake to recompress them to be slightly smaller. Figure out how big each episode can be to fit on the number of discs you want, then target that file size.

If you want to make Video DVDs out of them, then realistically you are limited to the standard capacities. 2-3 hours for a single-layer DVD, 4-6 hours for a dual-layer DVD (obviously, more time for higher compression/less quality.)

iDVD can recompress the movies to fit 3.5 hours on a single dual-layer disc; Toast can fit more.

 

I would seriously check out Toast Titanium, you have a lot of control on how you can burn your DVDs. I highly recommend it.