U2520D, MacBook, no scaling option

@andreaultrasharp  Scaling options on offer are entirely dependent on the OS, not the display.  Windows allows you to set scaling on any display type you want.  macOS is designed for "standard" displays of 100-110 pixels per inch, and Retina displays of 200-220 pixels per inch because those are the only classes of displays that Apple themselves have made, and Apple designs their software for their hardware.  The U2520D is a 25" QHD display, giving it a pixel density of 118.  So macOS probably considers it a standard display and therefore isn't offering scaling options.  You can see if this method works on your Mac, otherwise look into utilities like SwitchResX that give you a lot more options for scaling than macOS normally does.

@andreaultrasharp  Just to add to my post above, even if you get scaling working, be prepared for the display to look a bit fuzzy.  The reason is that macOS will only render natively at 100% (what you're using now) or 200% (Retina), because macOS has two classes of art assets: regular and "@2x" versions.  This again is related to the fact that those are the categories of displays that Apple themselves decided to build in their own products.  Having only those two options actually made it a lot easier for third party software to support Retina, which is why macOS has worked really well with Retina displays for years, while many Windows applications still have scaling problems because Windows is designed to support a much broader range of hardware.  But the challenge you'll have here is that you're using a display that isn't really within the range that Apple designed for.  As you're seeing, standard rendering is too small, but having macOS use the "@2x" Retina assets at full size would make things far too LARGE on your display.  That leaves the scaling options, but the way those work is that macOS renders internally using either the standard or Retina assets and then uses post-render GPU scaling to shrink down or blow up the resulting image.  That is going to introduce blurriness, especially on text.  The only real fix to this would be to get a display with a pixel density closer to the range that Apple actually designs for, like a 27" QHD display, which has a density of 108 ppi.

Incidentally, people who run 27" 4K displays with Macs have an even bigger version of this problem.  A 27" 4K display has a density of 163 ppi, which is basically right in the middle of the two classes that Apple designs for.  That means that standard and Retina are both equally bad fits and whichever you use, you're going to need a lot of scaling -- and the more scaling you need, the more blurriness you'll get.

You might be able to see text scaling options if you use a HDMI cable + Adapter. 

I have the same monitor and this is the only way I can see all possible resolutions, including 3840 x 2160 (= 2x 1080p). If the scaling factor isn't at least 2x, you will not see it in the system preferences of macOS.

And by the way, this is not a macOS specific problem. With a HDMI cable, I can see the same resolutions under Windows 10. I am yet to see the 'maximal resolution' of this display under Windows 10 via USB-C cable.

@mdas  The native resolution of the U2520D is 2560x1440.  If you're not able to see what when using a USB-C cable, I'm not sure what's going on.  But if you're expecting to see 3840x2160, the only way that would be possible with this display would be if either Windows rendered at that resolution (the desktop resolution) and then used the GPU to downscale the rendered output to the native resolution of the display (the active signal resolution), or if the display itself supported receiving a 4K signal and downscaled it internally.  But either way, you'll end up with scaling artifacts because the pixels of a 3840x2160 rendered image do not map cleanly onto a 2560x1440 pixel grid, so I'm not sure why you'd want to run that setup even if it were available.  The only scenario I've seen discussed here that kind of makes sense to me is when connecting a game console that only supports 1080p and 4K output (but not QHD output) to a QHD display.  In that case, 4K downscaled to QHD might look better than 1080p upscaled to QHD, so display support for receiving a 4K signal even though it can't truly display it can be useful.  (But even more useful would be if the game console supported native QHD output.)

@mdas  Ok, but none of that really explains what you meant about not being able to see the display's "maximal resolution" in Windows.  As for QHD being uncomfortable to read, that's probably why QHD resolution is much more commonly found on 27" displays than 24-25" displays.  I don't even understand why 24-25" QHD exists.  It results in a pixel density that's too high to use without scaling, and the artifacts and application problems that can come with that (especially on Windows), but not high enough to give a Retina-type experience of very sharp text.  A 27" 4K display is even worse, especially on the Mac side, since at 163 ppi it sits directly between the two "classes" of displays that Apple optimizes for, namely standard (100-110 ppi) and Retina (200-220 ppi) -- which means both of them will deliver an equally bad experience.

What I meant is I couldn't test how USB-C behaves with a Windows computer. The Desktop I could use to test it with has no USB-C port. 

If connected via USB-C (with a cable enabling high resolution & high data speed and so on...), would a Windows computer show 3840x2160? That would help me to know if I need a different cable or not. 

@mdas  I can't be absolutely certain since I haven't used a U2520D, but I would not expect 3840x2160 to be offered as an option.  In my earlier post I explained how it could be possible to send a 3840x2160 image to the display, but as I said there, I'm not sure why you would want to do that anyway if you're working with a system that could instead send a signal matching the display's native resolution of 2560x1440.  The only use case I can see for sending a 4K signal to a QHD display is if your source device only offers 1080p and 4K output options and can't send QHD.  But a Windows PC of course doesn't have that limitation.

I'm gonna test it tomorrow with a different cable and report back.

Well, I want the MacBook to send 3860x2160 to the monitor because macOS will show me the option to scale text to 'look like1080p' , which looks much better. Better than changing the resolution to 1080p. I thought I made it clear in the other post. 

1440p in macOS makes small text a pain to read. Sadly there is no option to change the font size systemwide without changing resolution, as for example Linux does.

@mdas  I don't know if this display will accept a 3840x2160 signal given that it only has a native resolution of 2560x1440.  So while I understand what you want and why, I don't know if that's actually achievable.  But if you find text difficult to read, then like I said earlier, the fact that you're using QHD on a 25" display rather than the much more common 27" size for that resolution certainly isn't helping.

The same issue is actual for me. Using of the monitor is not possible on macOS since all fonts are too small. 

The issue is not repeatable on other similar monitors. Looks like something is missed on the driver side.