All most all of the online advertising schemes, contextual or otherwise, today offer various ad formats such as Banner, Half Banner, Leaderboard etc, that come in various sizes. These formats and sizes are in fact defined by a standard (or rather a guideline) published by the Interactive Advertising Bureau (IAB).
The IAB is an organization comprising of over 375 media and technology companies, who have got together with the objectives of improving the field of online advertising and marketing.
This guideline, as stated by IAB, “provide a framework for advertising inventory and web page design”. Advertising networks, advertisers, publishers and web site designers, all benefit from the adaptation of these guidelines. Ad networks don't have to offer a multitude of ad sizes, advertisers can design their ads to fit these standard sizes, web site designers can optimize their template designs and finally, publishers get the flexibility of working with multiple ad networks without having to change ad placeholders.
The guideline defines standard names and sizes (e.g. Banner is 468 x 60 pixels, Leaderboard is 728 x 90 pixels), a maximum recommend download size, and for animated ad units, a recommended animation duration. In addition for ad unit guidelines, IAB defines guidelines for other forms of advertising such as pop-ups and rich media.
As publishers, it is wise to evaluate whether an ad network conform to these voluntary guidelines before deciding to sign up with that network.