For the easy solution, in iDVD, you have to go to the Project Info Window (
Project|Project Info... menu) and select a Double-Layer disc in the
DVD Type combobox.
A single layer disc is 4,2GB and an untouched 5GB movie file will obviously not fit on it.
If you don't have a double-layer capable burner, you will have to first reduce the quality of the video to reduce its size (through the widely used (and easy to use) Handbrake for example), putting the target size to circa 4GB. You will then be able to burn your file with iDVD or any other DVD authoring tool.
iDVD control of video file compression is too simple.
In iMovie, in
Share|Export You have several size options. Putting the mouse pointer on the little information symbol at the end of each line will give you the approximative size of the output file. I don't know if there is a finer control of output. I usually do it via Handbrake, as stated above.