If you use the Best Performance option in the Encoder Settings section of iDVD Preferences, you can use up to 2 hours of video in your double-layer project. If you select the Best Quality option, you can fit up to 4 hours of video on one Double-Layer DVD. If you wish to burn a single-layer DVD in one of the Double-Layer-supported SuperDrives, select the 4 GB (Single Layer) option in the Project info window in iDVD.
If your computer doesn't have one of the new SuperDrives installed, you'll be limited to single-layer DVD creation. You should only create Double-Layer disc images for testing purposes (it is only an option if you have a new SuperDrive installed). Burning those disc images to a DVD using a utility outside of iDVD will not set the layer break according to the DVD specification and may result in playback issues.
Note: If the amount of data in your project does not require a second layer, iDVD will ask if you wish to use a single-layer recordable DVD instead of recording the project on a Double-Layer disc.
Not all computers or DVD players will be able to read a DVD+R DL disc. See "Not all DVD players support all media types" for more information.