screen size has nothing to do with gaming performance, this is a common misconception, the resolution at which you play determines the quality of gameplay, i have a 21.5 in monitor and i can play at 800x600, this would be very low quality but would give me very good performance, whereas i could also play at 1920x1080, which has much better visual clarity but at the price of performance, i would say if you turned down the draw distance (distance at which details are present) as well as the traffic and the shadows you should be able to play at a moderate resolution, 1280x1024