I would recommend at least making your previews the resolution of your main display so that you can see the images full-screen when your referenced files are not connected (btw, the resolution of your display will have an asterisk beside it in the preview size drop-down box).
I have my preview size set to 1920 (or maybe even the size up from that -- I can't remember for sure) so that I can easily create slideshows for my HDTV if I want. But here's the catch, I also chose to disable previews for new projects in the Aperture preferences, and also turned off "maintain previews" for all projects in the library. I only create previews for my "better" images. The reason for this is that I'm still very much an amateur photographer who relies on taking multiple shots, stacking them, and then rating and selecting favourites for sharing and personal use. I didn't want my hard drive filling up with thousands of previews that will never be used for anything.
Food for thought!