Duncan Riley posted on ad sizes and placement over at the Blog Herald. Personally, I think a three column format, with ads (sky) on the left and menu on the right can’t be beat (and of course, put a medium or large rectangle in between the post and the comment box). But Duncan hasn’t done too badly either. I believe the most important thing is to keep ad placement in mind before you design your blog. If you’re blogging for dollars, good, prominent ad placement needs to be a priority in the design.