I am developing on a large monitor and would like to see, at a glance, how different parts of my application look at different screen resolutions.
I'm not interested in a utility which resizes my application or windows inside my application; I'm more interested in some sort of 2D screen overlay that can be set to different dimensions, and which I can move around the screen.
I have seen plenty of screen rulers, but I'd rather not have the hassle of measuring horizontally, then flipping the ruler, then measuring vertically. I'd just like a transparent, floating "box" that I can move around the screen so that I can tell, for example, if my dialogs are getting too tall.