ad size is a property of:
The size can be:
Based on experience and reading, not science.
See the iab specifications on IAB Standard Ad Unit Specification:
Full list:
Google - AdManager (Gam | Dfp )
Out of page:
More … Doc
https://developers.google.com/doubleclick-gpt/guides/ad-sizes#responsive_ads
With the devtool, you can inspect the size of an element.
Example with a wide scraper (160*600)
Ad Size in Advertising - Google Publisher Tag, see https://developers.google.com/doubleclick-gpt/guides/ad-sizes