Give me your experience, what size of ads are the most attractive. On which size the most users, visitors will click. My experience - ad 250*250, links 468*16 Yours ?!
Depends on the site. Cant generalise but I have found the 336- large rectangle, 728leader board and 160x360 skyscrapper to be good on diff. sites. Even 234 half banner proved really good when really blended well with the content. Rather than the size, the positioning is most vital, IMHO.
300 x 250 seems to work best always. Also, 336 x 280. Google itself shows in its guidelines that 336 x 280 ads have the highest CTR potential.
I've had good CTR with the large rectangle and large skyscraper. The skyscraper is placed in with the content, right beside a list of downloads relating to the site's topic. They are also both contrasting the color theme of the site.