ATC radar screens are usually square or round in shape, to represent displacements in the horizontal and vertical directions in a consistent manner. Computer monitors, on the other hand, vary widely in size, shape and screen resolution. To obviate the difficulty in projecting a consistent image onto differently-shaped screens, the Aspect Ratio (i.e. the ratio between the horizontal and vertical dimensions of the monitor) may be changed.
The best means of determining if your aspect ratio is set correctly is to look at a circular shape on the simulator and check it for correctness. VV Training courses will supply a map for this purpose with details contained in lesson material.
The Aspect Ratio may be changed in the Display Tab of the Setup Window.