What is a preferred monitor size?
Preferred monitor size is defined by the Monitor standards group (vesa) as the resolution as“The display manufacturer defines the “Preferred Timing Mode (PTM)” as the video timingmode that will produce the best quality image on the display’s viewing screen. The display manufacturerdefines the meaning of the words “best quality image”. For most flat panel displays (FPD), the preferredtiming mode will be the panel’s “native timing” based on its “native pixel format”.” 
Why should I use a monitor’s Preferred Timing Mode / Native Pixel Format?
Most LCD flat panel monitor users do not realize that operating their monitor in a resolution other than the native resolution negatively impacts the performance of their flat panel monitor. Unlike cathode ray tube (CRT) displays, the liquid crystal display (LCD) panel used in flat panel monitors has a fixed, predefined pixel format over a set area. A fixed pixel-format means that if the native resolution of the LCD panel is 1,024 x 768, there are exactly 1,024 pixels in each horizontal line and 768 pixels in each vertical line or 786,432 total pixels. When the PC’s resolution is set to the native pixel format, this allows for a 1:1 ratio and yield the best results. When the non-native resolution such as the 800 x 600 is displayed on a 1024 x 768 native resolution monitor scaling occurs. Scaling adjusted the image to fill the entire monitor screen. This results in manipulation of pixels to fill screen but, the manipulation may cause text and images to appear blurry.  There is no good way to show a non-native resolution on a fixed matrix display. 
How do I know what the native monitor size is for my monitor?
Monitor Information Reporting (MIR) v3 displays and collects this information.