There is a few ways.
1) Set window size every loop preventing resizing (not reccommended).
2) Get window size and resize contents (images) accordingly. See:
http://www.darkbasicpro.com/apollo/view.php?t=11701&b=1
3) Get window size and resize other dimension to make it square. For example, if user resizes x, resize y too.
4) Allow user to resize, then have an option to square the window. (from my expirience, this works best)
Hope that helps.
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