In Ubuntu 17.10 and Ubuntu 18.04, and Ubuntu 18.10, and Ubuntu 19.04 Settings > Devices > Displays offers no means by which the user can scale his desktop larger or smaller at a given resolution.
I always like to run my desktop at the highest resolution the monitor can handle (for clarity's sake), while at the same time I use desktop scaling to enlarge everything (not just font-scaling 1990's foolishness -- Or decrease my resolution foolishness).
The Unity desktop was decently granular in this regard. It didn't just offer huge jumps in scale-size. You could scale to .50, .75, 1, 1.25, 1.35, 2, 3, etc. You should see Unity on my 4k TV . . . so big clear and awesome.
Windows 10 (not quite as good as the Unity Desktop), also offers multiple scale sizes.
Ideally, I'd even like the ability to type in a custom scale-size up to 3 decimals out. For example, I want to scale my desktop to exactly to 1.347. Its 2017! Let's give the user the ability to fine tune desktop scaling preferences right in the GUI Settings (default).
It looks like I'll be on Ubuntu 16.04 until end of life.
18.04 does not allow fractional scaling in Xorg mode.