Discussion thread on the list.
Dimensions
For the side-bar spaces 125 to 140 pixels wide seems to work pretty well; ~125 pixels tall for a small button (those amazon buttons on the right sidebar are 125x125), ~600 pixels tall for a vertical banner. Probably on the order of 1000 pixels wide for the footer, maybe 90 or 100 pixels tall. For book/product review kinds of posts: the amazon text+image ads are width=120, height=240.
Site Ads
Post Ads
Left Sidebar Large Vertical
Left Sidebar Button
Right Sidebar Wide Button
Footer Banner