Systems and methods for automatically determining a content item size may be based on a size of a viewport and a width of a parent element. A script may be configured to determine a size of a viewport, determine a width of a parent element of a resource, and determine a content item size based, at least in part, on the size of the view port and the width of the parent element. A dimension of the determined content item size may be used by a content item selection system to determine a set of content items. A content item selection system may select a content item from the determined set of content items and serve data to effect display of the selected content item in the parent element with the resource.