Thanks for the note @CheyneHadley Unfortunately I do see the change you're talking about. I will check with the team to see if we can track down what happened. The good news is that I see the dialog boxes that you mention (Add Data, Save As, browse to folder) maintaining their sizes in a daily build of Pro 3.0. I'll see if there is any way to address this regression in an upcoming patch - there may not be, but we'll see.
As for the larger request of remembering the size and location of other properties dialogs like you mention, please add your support for this idea Remember last positioned window locations and size... - Esri Community