This is so easy, but you need a couple of free open source tools to get the job done. Get a copy of Handbrake and VideoLan (VLC Player) both 32bit. After installing these tools the only other tool you need is included with OS X 10.5+ and is called iDVD.

Insert you DVD and open Handbrake. On the menu bar toggle the Presets. I usually choose the Regular, High Profile and rip to the Movies folder as iDVD looks there. 

Once your movie is done ripping open iDVD and select your movie, I keep the defaults - wide screen Revolution theme. Once the movie is in iDVD all you need to do is make the new DVD using a standard 4gb DVD after tweaking the final video size. Open Project Info in the Project menu. Here you can select a Single Layer DVD @ 4.2g and Professional quality, or if you have a Dual Layer Burner, the 8g DL DVD. If you're in the US you should choose NTSC for Video Mode also, otherwise PAL.

Hit the Burn Project To DVD button and give it a hour or two. You new DVD should play on most modern DVD players.

Alternatily after ripping the video from DVD using HandBrake you can use Burn to burn to disk and not use iDVD. It will convert the video to mpg and burn easily. Great tool!

If you really like DVD Shrink you can use Wine Bottler to get DVD Shrink to run on Mac OS X. Simply download the DVD Shrink setup.exe and open Wine Bottler. Select create "Custom Prefixes" and select the setup file. If all goes well you will end up with DVD Shrink as a .app on your desktop. Has been working fine for me on 10.6 and will require X11.