For reference I'm at 25" 1440p with a font size of 9 using Consolas. Things look pretty crisp at native scaling and also very comfortable to read with no strain.
In terms of viewing space this fits (4) side by side code windows at 80 characters with a little bit of breathing room.
Yep, 27" 4k at 1.5x (plasma) I can have 50/50 or 60/40 with font size 14 and get 110+ characters, not just 80. 60/40 would be for an IDE with a bar or two on the side and still get almost 110 columns in the editor/console comfortably without eye strain. With 144 DPI fonts on plasma (using Hack), fonts seem plenty crisp to me down to size 5.
For screen size, I think there is some screen size that's most comfortable to view, and I want to say that's like ~40% of the human FOV... so, I think a 24" monitor and 15" laptop are most comfortable for me, but 27" might be a bit nicer if you're using it for media as well.
I miss read a bit when replying in the other comment, for 4 side-by-side 80 column windows, you'd need to bring the font size to 9, which isn't as comfortable as 14 for me in my side-by-side setup, but then 1.25x is also an option.
In terms of viewing space this fits (4) side by side code windows at 80 characters with a little bit of breathing room.