https://pxcalc.com/21.5" 1920x1080
DPI: 102.46
Dot Pitch: 0.2479 mm
The closest thing to what you’re currently staring at is
27" 2560x1440
DPI: 108.79
Dot Pitch: 0.2335 mm
That setup essentially gives you slightly smaller icons and windows than the ones you're squinting at right now.
If you're looking for the specific scenario that seems to be "troubling" you, it looks like this:
27" 3840x2160
DPI: 163.18
Dot Pitch: 0.1557 mm
30" 3840x2160
DPI: 146.86
Dot Pitch: 0.173 mm
Attempting to use those without OS scaling—which is just an invitation for visual chaos—is practically impossible for most people.
If your goal is to maintain almost the exact same object size on screen while jumping up to 4K resolution, you would need something along these lines:
42" 3840x2160
DPI: 104.9
Dot Pitch: 0.2421 mm
But you can forget about that option for obvious reasons.😁
Now, if you want to run windows side-by-side without cluttering your desk with a dual-monitor setup, an ultrawide QHD is your only real path; they usually sit at 3440x1440p with a 34" diagonal, which yields this:
34" 3440x1440
DPI: 109.68
Dot Pitch: 0.2316 mm
It’s a decent middle ground, provided you don't mind the fact that most of these panels use VA technology, which means you'll likely deal with frustrating black crush.
Did that clear things up for you? No? Fine.😁
Just buy two monitors or stick to a single 27" 2560x1440.