While determining which size ads I wanted to prioritize for one of our online marketing clients, I found the Most Popular Ad Sizes as determined by Nielsen/NetRatings.

Times change and as large screen monitors and larger resolutions come out, more and more advertisers(25%) are headed to the largest ad size- a 728×90 Leaderboard.  These are the large rectangular ads that are usually at the very top header position on a website.

Also popular with 16% of advertisers is the Medium Rectangle (300×250).  These have made popular with many sites that like to place an ad block within their content, usually between the article or page title and the page text.

Rounding out the top three is the Wide Skyscraper with 13% of advertisers using this size.  These ads usually display in a left or right column beside the main content of the site and reach quite a ways down the page.

Of course, knowing your target ad venue is more important than determining what is most used by other advertisers.  If the perfect site to advertise to your specific market only allows for Full Banner (468×60, originally the most popular size) or a Square Button (125×125) then you would need to accommodate their ad specifications in order to advertise on their website.