Use the native resolution for the monitor. If you do not know what that is or cannot find the information at the manufacturers WEB site, post up the make and model of the monitor.
Alternately, make sure the model and make of the monitor is shown in the display properties of the video settings in Windows. If it is, then the game will show you what resolution options there are for that monitor. That list is what the monitor told the video card is available. To deviate from that could damage the monitor.