I just installed RStudio on my laptop. It is a Yoga 710 with a small screen, running Windows 10. The default window size is larger than my screen size, so I can only run the program in full screen mode. Worse yet, the default font is way too big. I can't change it because the dialog window I get when I try to change Global Options is so large that the button that saves new settings is off the screen.
You can use Ctrl + - to toggle the IDE zoom level. Does that help?
What is your Windows display scaling set at?