PLEASE make it larger than a standard DVD. Even if you do not have a dual layered burner, you can always burn the movie over 2 DVDs.
This way, you can still use your normal burner until you can buy a dual layered. Why sacrifice quality?
Dual-layered burners are around $100 and the media is $10-$12 per DVD. It will all come down in time.
I would hate for you to put it in all the work and then realize the quality could have been much better.
Just put it on 2 DVDs for now.