At present there is no support for customizing the image width when the SizeForm is set to Large and we consider it as a feature request.
Please review the following description of the feature and let us know if it confirms to your requirement:
Customization support for Image Width in RIbbonButton
· Need to set the Image width using a property, for example LargeImageWidth
· Property default value is 32 and this value should be applied only when SizeForm is Large.
If our definition of this feature is different from your requirement then share the specific details, so that we can work towards fulfilling your requirements.