Why does changing Ad Format/Dimensions sometimes not change the size of the ad?
For some ad networks (e.g. WidgetBucks, Adroll, etc.) the dimensions of ads are managed through the online interface. There is no way to change these settings from within the WordPress system that would work reliably. You do not have to update these dimension settings if you update your Ad online, however, it can be useful in correctly identifying ‘Alternate Ads’ for AdSense blocks.