Animation menu --> Make Movie --> Quality --> Current Render Settings

And be sure to use a good codec when Poser asks about compression; Cinepak and Microsoft Video 1 are not good choices. For maximum quality, you'll need something lossless (like Huffyuv), or no compression at all. You can always compress the finished video later with a third-party utility. I'd also recommend setting Poser to render an image sequence rather than an AVI. If something should go wrong and Poser crashes or locks up, the incomplete video will be corrupted and unusable, whereas the image sequences will be fine (right up until the last frame rendered) and you can continue rendering from there. The images can be compiled into an AVI using a third-party utility like VirtualDub, Jasc Animation Shop, or TMPGEnc.