1.2.673 is the development preview.
I wasn't aware of the keyboard shortcuts for font size or I would have tried them. I imagine they would have worked. The issue in this case is that had I even thought to look, it would have hard to discover them for the same reason it was hard to use the options dialog. (Of course, yes, I could have googled 
Thanks for the feature requests link, I hadn't seen that. With the ongoing evolution of the RStudio site I've lost track of what's where.
This would not be a high priority enhancement, but I thought it worth mentioning.