1. A method of selecting a format in which to serve a content item in response to a request, the method comprising: collecting data regarding activity related to content items served in response to content requests; for each of a plurality of formats, using a processor to calculate a performance of the format when one or more content items are served with the format to a specified destination during a specified window of time; receiving a new content request for a content item to be served to the specified destination, the new content request having a set of attributes; and only after receiving the new content request: based on the calculated performances, a size of the requested content item, and a visibility with which the requested content item will be displayed, selecting a first format to serve in response to the new content request; and based at least in part on the attributes, selecting a first content item to serve with the first format.