Help me understand: movie length vs. GB size

Project size does not matter. It's total playing time that is important. iDVD encodes (compresses) the project to fit on the disc. There are 3 encoding settings the user can use:

Best Performance - projects with playing time up to 60 minutes or less, including menus.

High Quality - for projects up to 2 hours of playing time or less, including menus.

Professional Quality - f for projects up to 2 hours of playing time or less, including menus. This setting is a 2 pass setting. The first pass determines which parts of the project can sustain the most compression without quality loss (still photo slideshows and parts of movies with little movement). The second pass then compresses those areas accordingly.

More about that can be found here: iDVD '09 (7.x): About iDVD encoding settings

Follow this workflow to help assure the best qualty video DVD:

Once you have the project as you want it save it as a disk image via the File ➙ Save as Disk Image menu option. This will separate the encoding process from the burn process.

To check the encoding mount the disk image, launch DVD Player and play it. If it plays OK with DVD Player the encoding is good.

Then burn to disk with Disk Utility or Toast at the slowest speed available (2x-4x) to assure the best burn quality. Always use top quality media: Verbatim, Maxell or Taiyo Yuden DVD-R are the most recommended in these forums.

Just to add to OT's great advice:

iDVD encoding settings:

Short version:

Best Performance is for videos of up to 60 minutes

Best Quality is for videos of up to 120 minutes

Professional Quality is also for up to 120 minutes but even higher quality (and takes much longer)

That was for single-layer DVDs. Double these numbers for dual-layer DVDs.

Professional Quality: The Professional Quality option uses advanced two-pass technology to encode your video (The first pass determines which parts of the movie can be given greater compresson without quality loss and which parts can’t. The second pass then encodes those different parts accordingly) , resulting in the best quality of video possible on your burned DVD. You can select this option regardless of your project’s duration (up to 2 hours of video for a single-layer disc and 4 hours for a double-layer disc). Because Professional Quality encoding is time-consuming (requiring about twice as much time to encode a project as the High Quality option, for example) choose it only if you are not concerned about the time taken.

In both cases the maximum length includes titles, transitions and effects etc. Allow about 15 minutes for these.

You can use the amount of video in your project as a rough determination of which method to choose. If your project has an hour or less of video (for a single-layer disc), choose Best Performance. If it has between 1 and 2 hours of video (for a single-layer disc), choose High Quality. If you want the best possible encoding quality for projects that are up to 2 hours (for a single-layer disc), choose Professional Quality. This option takes about twice as long as the High Quality option, so select it only if time is not an issue for you.

Use the Capacity meter in the Project Info window (choose Project > Project Info) to determine how many minutes of video your project contains.

NOTE: With the Best Performance setting, you can turn background encoding off by choosing Advanced > “Encode in Background.” The checkmark is removed to show it’s no longer selected. Turning off background encoding can help performance if your system seems sluggish.

And whilst checking these settings in iDVD Preferences, make sure that the settings for NTSC/PAL and DV/DV Widescreen are also what you want.

http://support.apple.com/kb/HT1502?viewlocale=en_US

To maximise what quality is there, I would chose Professional Quality regardless of the length.

Keep the 4:3 aspect ratio - just set your TV to that format when you show them.

Double click on the .dmg file so you get this on your Desktop:

and the contents look like this:

Then you can play the project with DVD Player.