I do that on 27" monitor that has native 2560x1440 and when I set it to 1600x900 windowed mir is just about comfortable size.
If you start mir in vmware player in full screen mode, you effectively get the windowed mir (in the player window) but if you resize the window of the player in any way by pulling on the window or maximizing it, the buttons focus changes place (like 'hot image' focus spots shift to a different place if you know about website building) and where click works is different from where the button you wanted to click is located, the mapping is thrown off. So that's not the way to do it, just resize the res to suit like koriban says.