The opening screen will take you to the main menu where you should click on the screen symmetry button. This will allow you to adjust your monitor so that round and square objects are actually just as tall as wide. Once adjusted, you must 'Save' the calibration and then you are allowed to continue. This will only need to be done once unless you change monitors, change the patient distance, or change the resolution of the current monitor. The higher the resolution, the sharper the optotypes will be.
Larger monitors are better also.
A 17 inch is obviously better than a 15. The patient distance is directly related to the size of the optotype. You may experiment with different patient distances to find acceptable quality optotype with your particular monitor and patient distance desired. You should also use a monitor with a high refresh rate to avoid flicker of the screen especially under fluorescent lighting. You have several options for hardware setup.