DVD format is MPEG2. Your video must be MPEG2 to play as a DVD.
If you drop some other format into DVDA it will rerender it to MPEG2 and tell you how much room there is. However DVDA seems to be famous for overestimating the size of the video. My advice would be to render your video into MPEG2 with your editing program and take that to DVDA. Ideally render to MainConcept DVDA compatable MPEG2 (video) and ac3 (audio). Then DVDA will not have to rerender anything and you should know what your sizes are.