Hi and welcome to the forum
Providing that you are trying to do a backup of a movie that you own, most of video DVD are bigger than a single layer disc, so you have two possibilities.
1) Compress the original dvd or remove all undesidered stuff with a software like dvd shrink or clonedvd
2) Burn a 1:1 copy of the original DVD on a dual layer disc
Dual layer discs have higher prices than single layer discs, so is a your choice.