You must have righted clicked at some time and told the Open screen to Maximize.
Right click on the Open in the top left corner of the screen and click on restore and you will find it returns to normal size.
When I click on the "Open" folder icon or go to "File", "Open" on the tool bar, the window that opens with the lists of all my drives, desktop, etc. takes up the whole screen (it used to be a smaller/normal size). I have tried to use the mouse pointer to snag one of the corners of the window to drag it and make it smaller with no luck. There is no "minimize", "restore" or "close" options on the window. It's a minor issue; but is really annoying. Any ideas?