What Sizes Do I Use For Custom Size Banner?
These can be any size you want. Ideally they will be all the same size, but you have some leeway there to experiment and use different sized banners.
If you do, you should only use these campaigns on your own sites, as other website owners will likely block you, if your banners come up different sizes all the time. Or else they’ll show them resized and possibly distorted to fit their own formatting.