In the advertising world there are some very specific ad banner sizes laid out by the IAB to help streamline the ad creation/submission and site design processes.
http://www.iab.net/iab_products_and_...1421/1443/1452
http://www.iab.net/iab_products_and_...76/508767/1461
http://www.iab.net/iab_products_and_...767/Rich_Media
If you look at every mainstream media outlet they are running some combination of these banners.
However, as I'm grabbing banners from the various affiliate programs I'm finding that very few even support the most basic of standard sizes and instead have dimensions that seem to have just been randomly chosen.
Why?