Ok, I'm going to go ahead and assume you're talking about making an SD DVD as opposed to an HD-DVD or Blu-ray.
Given the fact that your final DVD will be 720x480 in size, your Motion graphics need not be any bigger than that. However, this doesn't mean that you should export your Motion project using a lossy codec like DV. Instead use the highest quality codec you can like Uncompressed 8 or 10bit.
DV codec is notoriously bad at handling gfx so stay away from it if you can.
Export a QT movie and then let Compressor do the MPEG compression. There are presets available for DVDs based on length of project.