There's a way of determining whether it will fit on your disc, but first you need to click on "Settings" then "Encoding" then make sure the correct target media size is selected and click ok. It should always use this setting until you change it. It doesn't matter if you close the program. Everytime you open it, it will be selected.
Anyways, moving on, When you load the files into the program, you should see a simple dialog display at the bottom left saying how many videos you have loaded, how long the total runtime of all videos, and how much space the files will take up on the media (DVD). If it is higher than 4.7GB then obviously it's not going to fit on a single layer DVD. Personally I would try to avoid close calls and just round down to 4.5GB and not go over when burning to single layer DVD, and 8GB for a dual layer DVD.But hey, that's just me.