Steve,
How long is the movie - in time? It depends on the flile and what compression it has been saved as. An Uncompressed file will be huge - I hour can almost be 10 gigs if I remember right. WHen you make a DVD in iDVD it will tell you how much room you have and or need. The easiest way is to use iDVD and import the movie in and it will tell you if it will work. If you have DVD Studio you cn drop it in there too and it will take the movie and compress it for you. DOn;t worry about the size of the file.

There are Tutorials on the Apple site.. http://www.apple.com/ilife/tutorials/#idvd

Jeff