How DFP determines ad unit sizes
Fixed size ad formats are displayed as width x height, in pixels (example: 250x85). An ad unit size is determined by the size of the ad intended to fill the unit.
With DoubleClick for Publishers, you can target smaller creatives to a larger ad unit (example: a 120x600 creative in a 160x600 ad unit) without the ad displaying extra white space. You can also target larger creatives to smaller ad units, but must select Allow creative size to differ from ad unit size if using an image or Flash creative.
When you define ads and ad units, we recommend that you follow the Google AdSense ad formats and abide by Interactive Advertising Bureau (IAB) standards. While IAB sizes are recommended, you can define any size you want for non-AdSense ads and ad units (AdSense creatives only permit IAB ad sizes, which you must observe if you've enabled an ad unit for AdSense).
DFP has some different sizes available for apps, such as Smart Banners. Learn more.