I'm running a blog about European Internet startups (in English). In the right sidebar I placed a small square (200x200) with image ads. Now, I'm planning to make the sidebar a little bit wider and place a square (250x250) there. I'm aware of the fact, that a medium rectangle (300x250) would be better - since it seems to be the more common format with more ad inventory (more competition / higher cpm). My problem: The medium rectangle would be too wide and doesn't fit to the overall-layout of the site.
Now my question: Is it worth it to change the small square to a square? I mean: Is there more ad invertory (+more competition) in the tech-space for the square then for the small square (or maybe less). Has anyone here already made some ​​experiences he/she could share?