Like others have said, it's the resolution. Sort that first, and then let the size follow to an extent.
I've had a few QHD(BenQ and Viewsonic 27" 2560x1440) and UW-QHD displays (34" 3440x1440).
For me, the 27" were a good step up from a standard 1080P/similar-ranged 24-27" display, and while I love the 34" LG 3440x1440 (have had a couple of years now), for the size, it's lacking vertical resolution, so am likely moving to a 38" 3840x1600 'QHD+' screen. That may still not be 'nirvana' for what I do and how I use it, but it should be pretty close, letting me do 2 app windows vertically in some cases, while currently most apps I use don't fit well 2 up vertically, although I have plenty of room horizontally.
If you're used to running a 1080P display, consider 4K is effectively a 2x2 grid of those, so consider if that is really going to be what you want (running 4 app windows for example), and at what single display size you might need to make use of that. In other words, if you have the 'right' resolution for what you want to do (i.e. 4 maximized apps on current display concurrently), but you get too small of a display, you'll need to play with font sizes or scaling in order to make things work, and then may not get the desired benefit.
I'm somewhat torn myself between finding (if they even exist) a pair of 30" 4K or 2560x1600 displays to run without scaling, add a second 34" QHD to my current setup, the 38" QHD+, or a single large-but-not-ultrawide 4K or 5K display.
Define your usage and what you'd really like to do more of (display more apps 'full size' side by side might consider ultrawide displays), etc..