getting the size of a output pane

I am writing a simple program that solves the traveling salesman program
by simulated annealing (Metropolis algorithm).
I wish to plot the cities on screen and the path between them.
To do this I wish to use the entire graphics area of the output-pane.
In order to scale this I need to know the extent of the window.

How do I get this area (width, height)?
(assuming I resize it)

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