The Creative sizes report shows you the sizes of the display ads (image, Flash, etc.) that have served on your site. All standard display ad sizes, e.g., 728x90, 336x280, etc., are shown in the report, while text ads and ads that don’t have a standard size are shown with the label “Dynamic”. Note that the ad size that was served on your site might not be the same as the ad size that was originally requested by your ad unit.
You might find the Creative sizes report particularly useful if you’re:
Using responsive ad units, which don’t have a preset requested ad size and are shown under the “Responsive” label in the Ad sizes report.
A Google Ad Manager publisher who uses AdSense for dynamic allocation. The Creative sizes report shows you dynamic allocation data which you can’t see in the Ad sizes report, since for dynamic allocation the ad requests aren’t made from an AdSense ad unit.