I've a LG 34UM95, an ultrawide monitor with a native resolution of 3440 x 1440 and in Substance Painter all seems too small to work.
In my case if I have the OS scaling to 125% and all looks tiny. If I put 150% all gets bigger. As someone said: there is no middle grounds.
Seems that you only has tested with 1080p monitors and 4k. But forgot to test with intermediate resolutions (2560 x 1440, 3440 x 1440, and so on).