I assume you are talking about the Discourse DFP plugin:
Be aware that support for additional sizes needs to be supported in JavaScript as well as CSS for DFP.
In addition you can have it dynamically select from several sizes based on the viewport size, more details here:
https://support.google.com/dfp_premium/answer/3423562?hl=en
This is more a development note and doesn’t provide you with something you can action unless you are a developer.
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