Size is an important attribute in banner ads. All the banners occupy rectangular spaces on web pages. Dimensions of such rectangular spaces are known as banner sizes. When you are putting your banner on other websites, you need to know what are the available spaces in the site. Based on the space availability you need to design your banner.
Internet Advertising Bureau (IAB) issued a list in 2001 where the (
http://www.iab.net/abouttheiab/recentpressreleases/pressreleasearchive/pressrelease/4403 ) standard banner sizes for the advertisers were prescribed. According to IAB