DVD standard is 720 x 576 (PAL) so you can't change that, but you can change the 'bit rate' which gives you control over the quality of the video and the total playing time.
Above is the 'Encoder settings...' window from the DVD burn menu. As you move the slider you can see the variable options. Easiest is to click on the adapt button and MEP will pick the best quality that can fit. Remember that menues and chapter stuff takes up some space as well. See more in your help file under 'encoder settings'.
Dual layer solution is probably the best solution without sacrifice to the quality but I not like it because DL disk is not a current stock on stores shelves also you need a burner to support dual layer and a player supporting DL.
The solution is to keep the standard format (720) and reduce to a minimum in the video the effect, zoom, plus cut any scene that fill the video and give nothing. Also you can reduce the baud rate or use the VBR (variable baud rate) function. Also you can cut the frame rate to 5 or 10 per second if you video contains less motion.
The best solution is to create 2 standard DVD if you need to change nothing to your configuration, less time and better result on playback. This is done automatically inside the software.