Be careful with terminology: I read your request (the dimensions of the monitor) as a request for physical dimensions, like a 19" flatscreen. I expect you really want pixel area, like 1280x1024 pixels.
Also be careful with assumptions: not all setups use just one monitor, and the combined surface area is not always a simple rectangle. Thus the best window dimensions for a given application is not at all consistent or obvious. Add to that the concerns for various trays and other edge-warts, and you may not even be able to accurately request the usable desktop region in one shot.
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