Lets say you connect a 27" UHD "4k" display (lets ignore 5k for the moment) and select your preferred display mode (lets say the highest "looks like 3840x2160" mode - the default "Best for Display" option on my 4k screen (may vary) - which, on a Mac, means the native, 1:1 resolution of UHD without any pixel doubling.
- You open up a document in Pages and it comes up, by default, in 125% zoom (it does - just tried it) - which is bit too small for your vision.
- You increase the zoom to 150% - aah, thats about right.
- How big a screen do you need to buy so that the default "125%" text comes out at that size in the same screen mode?
Well, you've increased the size of the text by 1.2 (125 x 1.2 = 150) so you'd get the same effect if, instead, you enlarged the screen by a factor of 1.2. 27" x 1.2 = 32.4".
So, you'd be looking at a 32" screen (which sounds about right to me if you want to
comfortably use a 4k screen in native mode).
(The people saying 40" missed that you
started with the text at
125% and
increased it to 150% - a x1.2
increase not the x1.5 increase you'd have needed to go from 100% to 150%).
Now, there's the issue of whether you're going to see pixels. Here you go:
https://designcompaniesranked.com/resources/is-this-retina/
Fill in 3840x2160 at 32" and that site will tell you that, at a viewing distance of 25" or more that is equivalent to 300 pixels-per-inch at 12" away - the rule of thumb for not seeing any pixels and Apple's definition of "Retina display". The calculation is done by converting the linear PPI into the angle between light rays from adjacent pixels as seen as your eye and assuming that the minimum
angular separation* that your eye can resolve is independent of distance.
Which works for a theoretical pinhole camera with no concept of focus (or astronomical instruments where the distances is always, for practical purposes, infinite). However, in the case of a biological ball of goo that focusses by contracting muscles around a squishy lens, and only produces a decent image thanks to an amazing neural-net image processing system alongside which an iPhone X looks about as smart as a light switch... well, basically, don't lose sleep over whether you're sitting 2" too close for the screen to be retina...
...anyway, although the visibility or not of pixels on your screen undoubtably does have an effect on the size of text that you can comfortably read, that's pretty subtle and not something that can be reduced to a scale factor.
Couple of added complications:
- There are virtually no 5k displays available beyond the iMac screen and the Apple/LG 27" ultrafine display.
- There are a few "true 4k" displays (4096x2304) but most are "UHD" 4k 3840x2160 and all will be slightly more "pixel-y" than a 5k display of the same size.
- You can't actually set an iMac into "1:1 looks like 5k (5120 x 2880)" mode (which would be pretty unusable at 27") - although the screen is always driven at 5120x2880 the OS applies a scale factor of (in default "best" mode) 2 to everything, making text, icons etc. the same physical size that they would appear on the old, non-retina 27" 2560x1440 display - just a lot sharper (provided their applications support retina mode).
- The Mac also offers a range of scaled "looks like NNNNxNNNN" modes. On an old non-retina display, any non-native resolution just got stretched or squashed by the display until it filled the screen and usually looked awful. With a 4k or 5k screen its completely different: the scaled modes all apply a scale factor of 2, render everything as if it a huge virtual screen twice the size of the "looks like" resolution and then use the computer's GPU to re-sample it at the actual native resolution of the connected display. While the result is slightly "soft" compared with the default "best " mode, you have to look very carefully to see the difference, and the extra load on the CPU and GPU is not going to bother a high-end iMac or an iMac Pro.
So, really, the answer is to get the screen the way you like it by selecting the appropriate "scaled" mode rather than fine tuning the size of the screen, and look at bigger screens as a way of getting more space rather than making stuff bigger.
* Explanation of angular size: