Wondering which ad sizes to run on your site? Marketing Sherpa posted this chart to show the click through rates on advertisements. Assuming the same CPC (Cost Per Click) revenue for each ad size, the 300×250 and 728×90 receive the highest click-thrus.

Obviously there are many other variables, such as ad placement above or below the fold etc. However this is a good starting point when deciding which types of ads to place.