I'm not sure if this is a bug or a problem with my system. It seems like a bug, because it happens with the RStudio Preview version but not the latest stable version (I list my system and software info below).
Here's the issue: When I first open an RStudio project on my computer, it looks like the picture below. The IDE panes take up only a quarter of the full RStudio application window and all of the text is correspondingly shrunken and barely readable.
I have a three-screen set-up (laptop built-in screen plus two external monitors). If I drag the RStudio application window to any other monitor, the problem goes away--the IDE panes expand to take up the full RStudio application window, as shown in the screenshot below--when the RStudio application window has been dragged about halfway into one of the other screens. I can then move the application window around as needed without a problem. But if I close the project and reopen it, the shrinking issue occurs again.
Dragging the screen is an easy fix, but I'd like to see if the underlying issue can be fixed, so that this doesn't happen in the first place.
System and software details are listed below. If it would be helpful, I'm happy to provide more information or run tests under various conditions to further investigate the problem.
System Info: Macbook Pro 15" 2018 running OSX 10.13.6.
Problem occurs with RStudio Version 1.2.1237 Build 1246 (7a011999) but not with Version 1.1.463.
Session info for the pictured project:
R version 3.5.2 (2018-12-20)
Platform: x86_64-apple-darwin15.6.0 (64-bit)
Running under: macOS High Sierra 10.13.6
Matrix products: default
attached base packages:
 stats graphics grDevices utils datasets methods base
loaded via a namespace (and not attached):
 compiler_3.5.2 assertthat_0.2.0 cli_1.0.1 tools_3.5.2
 withr_2.1.2 rstudioapi_0.9.0 crayon_1.3.4 packrat_0.5.0