I currently use Rstudio on SurfaceBook.
The OS used is Windows 10.
SurfaceBook has high screen resolution.
The character of Rstudio is very small and very inconvenient.
Applications like VisualStudio and GithubDesktop do not have this problem.
Have you tried "View" -> "Zoom In"? That increases the text size. I would expect it would be available as an option for you and may solve the "small text" issue.
As @cole says "View" -> "Zoom in" will fix it but it will not only increase the size of the text in the R-Studio panels like "File" and "Plots" and such, it will also increase the size of the fonts in the editor windows. So once you get the size text you want in the R-Studio panels go back and pick a smaller Editor Font size for the editing windows. You can do that in "Tools" -> "Global Options" -> Options
One "Zoom in" won't make a noticeable change in the text size of the "File" panel. Keep using "Zoom in" over and over until the text in "File" panel is the size you want.
Then go back and change the font size for the text editor to the size you want.