I would estimate the font size in my LR (and all Adobe windows) is a five. It's the same on all three of my computers, all of which are operating on Windows 8.
The last time I raised this issue, I was told to change the screen resolution -- pretty poor workaround considering all my other apps operate at the recommended screen resolution for the three computer and one additional monitors I use.
How can I increase the font size to make Adobe applications usable on my computers?
I also use Win 8.1 and I have a high DPI 13" screen that is at 1920x1080 resolution and it is satisfactory although small.
LR (5.7 current) will not support screens with greater resolution unless you have razor sharp eyes.
We just need to hope that the next release (6) whenever it comes has native support for high DPI screens.