Kristen A. answered 06/09/19
10+ Years as a Web Designer Specializing in Wordpress
Hi!
Since Google Ads and the Google Adsense programs are by far the largest advertising programs on the web, I would go with Google's sizes. Of note, Google now offers responsive ads for AdWords users and they automatically size the ads to fit the spaces. The top sizes recommended by Google for display banners are:
300 X 250 px
160 X 600 px
336 X 280 px
250 X 250 px
728 X 90 px
320 X 100 px (mobile)