Regarding composition settings, you’re stuck with 720×480 (in North America, at least) for DVD. If you don’t use that pixel resolution, it won’t play at all, or it won’t compress to the necessary codec. You have no choice in the matter.
DVDs must be encoded with the MPEG-2 codec, which is a compromise between image quality and data rate, allowing us to play videos on a removable disc on all DVD players.
That said, it’s not the best quality. You can try to squeeze the most out of it by having the highest-quality source possible before compressing to MPEG-2. If you’ve generated the images in AE, you can render your movie to the Animation (lossless) codec or a 10-bit codec before compressing to MPEG-2. However, if your source is, say, DV, then you’re stuck with the low quality of DV, which is further compressed when you compress to the necessary MPEG-2 codec.
Also, when you compress to MPEG-2, you can use a setting which is VBR (variable bitrate) with the highest data rate offered by the compressor. Check your compressor app’s settings. I recommend a specialized compression app such as Apple’s Compressor, or maybe Squeeze.
Does that make sense?