Open Bidding

Open bidding allows publishers to invite other third-party partners to compete for your inventory in a single auction with real-time server-to-machine bidding owner

Open bidding in GAM provides simpler management, reporting, and billing options. Eligible inventory managed in GAM can take advantage of open bidding without requiring publishers to implement any additional technical functionality. If a publisher is interested in improving ad revenue, then that might be the answer for you.

In Content Ads - Best Practice

300×250px — Medium Rectangle
728×90px — Leaderboard
320×50px — Mobile banner

These 3 sizes are very popular in programmatic due to high demand by advertisers and because they yield higher CPM.