You've effectively answered your own question with the image from this article, which discusses the importance of pixel density over size. The exact ranges may be be a matter of personal preference, but:
- PPI below 100 will generally look very blurry or pixellated; and the large pixels will make objects look large. A 1080p display at 27" or larger will fall into this group.
- PPI between 100 - 125 is a sweet spot: objects not too big, nor too small, with reasonable clarity.
- PPI between 125 to 200 has pixels so small that objects are tiny, but if you scale them down by 2x, you're back below 100ppi in terms of object size. You could use 'non-integer scaling' to scale things to a more suitable ppi.
- As per the article, using non-integer scalings (1.5x, 1.25x...) to simulate different resolutions can produce 'visual effects', as well as working the GPU harder. However, these may not be that noticeable, and GPUs are increasingly powerful these days. So a 4K 27" may work well enough at a scaled resolution (e.g. 2560x1440), if you already have one.
- PPI above 200 is where Apple's "Retina" displays sit, and these benefit from the clarity of the high-resolution pixels, which are then scaled down to the 'sweet spot' for object size of c. 110ppi.
For choice, a good bet is a 2k 27" display, at 109ppi; you'll likely have a lot of options in that size. Slightly higher ppi would be better, if possible.
I don't think the refresh rate is important, unless you're dealing with gaming or video where there's constant fast-moving images.
That having been said, I've recently bought a Samsung 24-inch 2K display:
- At 122ppi, I don't find it too small.
- It's obviously the same number of pixels as you'd get on a 2K 27" screen, but at 89% the size, which makes things sharper. It's as tall as an A4 page (portrait) at Actual size, and about 2.75 A4 (portrait) pages wide.
You may be able to find a larger display that's "cut from the same cloth" (e.g. the same pixel panel) in the factory.
It was less than £200, so you could probably buy two or three 2K 24" displays for much less money than a 5K display...!
TL;DR: Bigger isn't always better, and ppi is important.
One other parameter to be careful about when buying monitors is the power consumption. Newer displays tend to be more efficient; but slightly older tech (usually cheaper, or on 'special' offer) can be twice or even three times the wattage.
