My seated position to my 43" 4k screen is such that if I stretch my arm straight out in front of me the tips of my fingers can almost touch the screen. I have pretty long arms though. The distance to my eyeballs if probably 2 to 2.5 feet. It's what I consider a normal seated position in front of a Desktop monitor, but everyone's normal varies I guess.
From that range, I find that ~100 DPI is perfect. 90DPI is a little too low, and 110 DPI is a little too high.
It just so happens that classic desktop resolutions and screen sizes (like 1600x1200 at 20", or even the much newer 2560x1600 at 30") also wind up being almost exactly 100 DPI. Maybe it is that I am accustomed to those classic resolutions, and that's why I like it this way, or maybe it's because those classic resolutions were developed to some human eyeball science that I am unfamiliar with, but around 100DPI just winds up being perfect for my seated position.
I'll admit that the first time I sat infront of a 40+" 4k screen the large size took some getting used to. Then again, so did the first time I sat infront of my 22" 1600x1200 (20" visible) Iiyama CRT back in 2001, or the first time I sat in front of my 24" 1920x1200 Dell 2405FPW in ~2005, and my 30" Dell 2560x1600 screen in 2010. It turns out every time I have upgraded screen size in the last 20 years I have been a little overwhelmed at first, but then gotten used to it.
You wind up playing the game a little differently that way I think. By necessity more focus on the center, less on the corners which become "peripheral vision". It's probably not something you'd want if you play most competitive multi-player games, but in a visually stimulating story based single player FPS it can really just suck you in.
43" is perfect for me at 4K. 48" just a tad big, with just a tad too visible pixels. 40" would probably make me squint in day to day productivity use without scaling, and scaling always feels like kind of a kludge to me.
