The subject matter of this specification can be embodied in, among other things, a method that includes receiving a request for content to fill a block, the request including information about a size of the block and criteria for identifying eligible content items for inclusion in the block, determining a number of content items to be included in the block, including determining amounts of dynamic resizing to apply to content items, determining an estimate of a difference in conversion rates conditional on receiving an interaction in the different configurations that perform differing amounts of dynamic resizing, and applying the estimate when determining an efficiency for a number of permutations of allocations of eligible content items, establishing a price to be charged to each content sponsor associated with a permutation that has a highest efficiency, and providing the eligible content items and charging the content sponsors based on the established prices.