I'm using RStudio 1.4.1725 in Xubuntu 20.04 (same issue in earlier rstudio 1.4 versions). I have window scaling set to 2.0. The small menu bar font size problem is similar to that in this post, but not identical. The "New Folder" popup also exhibits the problem, but File Save (for example) is a system menu so it's fine.
Only two apps I'm using exhibit this problem (RStudio and globalprotect). Octave, Firefox, Chrome, emacs, libreoffice, gvim, teminals, mousepad, keepass, virt-manager, etc. are all fine, with appropriately sized menus that respond to system, app, or qt5 settings.
Apart from the top menu bar and New Folder menu, all other font sizes in Rstudio are adjustable. Any suggestions?