"This is identified behaviour to do with compatibility between the Windows Operation system automatically applying a high DPI sc.aling to ArcGIS Pro resulting in the behaviour you saw previously.
You can submit and idea on the Ideas website.
I would highly recommend that you create an request so that this behaviour can be addressed in future releases."
I don't understand the message I got. But I'm passing it on. ArcGIS Pro doesn't maximise to the full size of my screen's resolution.