I guess I'm not sure what you are asking. Below is a shot of MA3 in windowed mode stretched across my desktop.
OK. How do you change the Window Size? In window mode, grabbing the corners to stretch WILL NOT change the size which is displayed after the Timing Clamp and Sleep parameters. Where in the settings do you specify the window size? Display Settings is not the place to change the MA3 window size and I cannot find how to change it to the way your screen grab is showing. My window size is 708 x 449. Where do that come from? How to change it?