The smallest resolution used nowerdays for Windows PCs is
800x600 (especially 1024x600 on Netbooks for example)

People using older computers than that will not game anyway.

Nevertheless its smart to always place your GUI Panles according
to the screens corners (and not fixed positions) anyways,
to account for different widescreen formats.

The 3D World is rendered correctly anyways.

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A small 640x480 (or smaller) screen makes sense if you want to make
a desktop-window-mode game like a casual Mine-sweeper or card game.