You can try to check implementation of the function
func validBannerSizes(for adLoader: GADAdLoader) -> [NSValue]
It's part of GAMBannerAdLoaderDelegate
This function is responsible for setting sizes of the ad which you want to request from GAM
https://developers.google.com/admob/ios/api/reference/Protocols/GAMBannerAdLoaderDelegate#-validbannersizesforadloader: