My preferred setup is something small but manageable. By this I mean something that I can scale if needed but also reliable across different domains (such as gaming, coding, web surfing, etc)
How many?
2
Size?
~27inch
Resolution
2560x1440
Why not 4k? The 4k'ish feel of having multiple windows opened on your monitor will plenty of space left over is a good vibe. However, I've experienced several IDE's that do not scale well with 4k monitors. Several embedded design suites look awful because of this and you sometimes need a magnifying glass just to find the right button you need. Yea you can find work arounds for this but why deal with it at all.
Ultra Wide
They look appealing I admit but haven't invested in one yet. There's just something about multiple monitors that's appealing to me over one gigantic one. Plus if I ever have two systems I want to hook up and see at the same time then I have this option with two monitors. If I had one gigantic one i would need to invest in a switch i guess.
I don't like the glossy style screens. I like that dull rough look (I can't think of the technically term for it but the less shiny screen whatever that is). I use a few color schemes that just look awful on the glossy "shiny" style screen. My eyes start to hurt on those screens as well which is another reason I favor the "dull" screen look.
I get a high frame rate monitor. I find it less stressful on my eyes along with the dull screen I mentioned above. Comfortable distance is about 2 to 3 feet.