Ad size 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 DFP Small Business, you may 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 may 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. (AdSense creatives only permit IAB ad sizes, which you must observe if you've enabled an ad unit for AdSense.)