The logo or symbol part of the icon will have to have a version that looks good at 16 x 16 (really 15x15) pixels. Obviously, I want a cool, non-pixelated version for the full logo that will be on the website, but I'll need a tiny version as well, so the icon part of the logo can't be something that is distinguished only by tremendous detail.
More info see the "Icon Size and Placement" section at:
http://www.eclipse.org/articles/Article-UI-Guidelines/Contents.html