Hi David,
There does not appear to be a way to fully customize the popups in Pro, like you can in ArcGIS Online. So, that thumbnail size will stay that way, as far as I know.
However, you can make it clickable to add a hyperlink, etc. In the image options of the popup, you can specify the field or web address for the Hyperlink.
See this article for more info:
Configure pop-ups—ArcGIS Pro | ArcGIS Desktop