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Standard Animated Banner Sizes
source: http://www.domain-b.com/

Below are some of the standard animated gif banner sizes used on web pages. It is a sort of convention to develop banners based on these sizes; however, you are not bound by these dimensions, as it were. Two other sizes for animated gif banners are also in use: 350x300 and 720x300. These are generally not used on web pages but as pop-up or pop-under ads displayed in separate browser windows.