Macbook Air reading wrong size and resolution of external monitor

I have LG Flatron E1941 which has a screen size of 18.5" and native resolution of 1366 x 768 but when connecting to my Macbook Air 2017(macOS Catalina) it is showing 13.9 inch and 800 x 600 resolution in System Preferences > Displays. I am unable to find the desired resolution under displays settings as well, tried switchresx as well, but the quality in the desired resolution is also not good. d.