Deciding the screen size
You ought to consider the right physical size in inches with the screen resolution — deviating too much from the following chart would either make the screen too pixelated or you might find the default font size to be too small.
Screen Resolution | Size in inches --------------------------------|---------------- Full HD (1080p or 1920x1080px) | ~24" Quad HD (1440p or 2560x1440px) | 27"-32" 4K (2160p or 3840x2160px) | ~32"+
Gaming Vs. Regular Monitors
Gaming monitors typically have high refresh rate (upward of 100Hz). However, as of 2019, they sacrifice colour for refresh rate, and the screen looks rather bad for other tasks like programming, office work or photo/video editing. So for regular work, buy a 60Hz or 75Hz monitor. 60Hz is not a bad speed of gaming btw. Just 5 years, back that was pretty much what everyone was using.
Display Panel Type: LED (IPS, TN or VA), LCD etc.
For gaming monitors, IPS LED is sort of the successor to TN panels (as in better colors even at high refresh rates). VA panels have higher max brightness, better contrast, better colors etc. So great for office work but not for high speed video games (where they might have blurring).
I feel LCDs are easier on the eyes. LEDs are literally small diode bulbs directed towards your eyes, while in LCDs the backlight passes through a polariser glass, which cuts some of the glare. I never really go above 50% brightness on my monitors, so the extra bright LEDs seem sort of useless to me. But the era of LCDs seems to be gone now (except Apple still continues to bring some excellent LCD display tech with iPhone Xr lineup. Hope it continues!)