Because the brushes aren't actually designed to work above 100% and can show some odd behaviours when running at larger scale the upscale feature isn't available on the general slider. This is something that we will be looking at but it's part of the whole process of replacing the app and its interface with something high DPI aware, something that's not quite so straightforward for pixel based painting with a non-standard interface. What you've done there is tell Windows that ArtRage supports High DPI, which it technically doesn't, and while you could scale the panels to 200% in Preferences to try and make up for that, the actual solution involves basically rewriting the entire front end so it's not something that's going to happen overnight.