For long typing sessions, a 24-27 inch monitor at Full HD (1920×1080) or higher works best. Higher resolutions (2K, 4K) mean sharper text and less eye fatigue - at 27 inches 2K is worth considering, and at 32 inches 4K becomes almost mandatory.
The most common panel types are IPS and VA. IPS delivers natural colours and wide viewing angles - a good fit for most office work. VA gives deeper blacks but handles side lighting less well. Avoid older TN panels - they tire the eyes much faster and aren't worth buying anymore.
The screen surface should be matte, not glossy - it reflects less ambient light, so you don't have to squint when working near a window or under a lamp.
A standard 60 Hz is plenty for typing - higher values (120 Hz, 144 Hz) mostly matter to gamers. What matters more is whether the monitor uses flicker-free backlight, free of PWM flicker. A cheap panel without it can give you a headache after an hour of work, even if you can't see any flicker with the naked eye.
Most modern monitors come with a built-in low blue light mode. Switch it on if you work in the evening - the warmer colour temperature helps your eyes adapt to dimmer ambient light and noticeably reduces fatigue. Many operating systems offer the same thing (Night Light on Windows, Night Shift on macOS) - you can enable it independently of the monitor's own setting.
Whichever model you use, the top of the screen should sit at eye level and the screen itself about an arm's length away. Most monitors offer height, tilt and swivel adjustment - use it. If you work on a laptop, raise it on a stand or a stack of books and plug in a separate keyboard - hunching your neck over a laptop screen is the fastest route to neck pain.
Two monitors are increasingly the default - one for your main work, the second for reference windows or chat. If you plan a setup like this, buy two identical models at the same time. Different heights and colour profiles strain the eyes, because they have to re-adapt every time your gaze jumps between screens.