You should be getting a message to 'Automatically adjust the bitrate' - if you have already done this and the movie is still too large :-
1. Check that you have no blank space at the end of the movie or a rogue clip at the end of the timeline. If you have you can delete the excess by selecting a range from the actual end of the video to the end of the timeline and use the Range, Delete option
2. If the movie is really over 2 hours long then consider using a Dual Layer disc (assuming your burner supports DL discs) or splitting it up into two smaller movies.