I have a 2015 macbook pro with a retina screen running Windows 8.1. With no scaling, the UI on the retina screen is tiny.
The solution is to increase the scaling using the settings found in:
Control Panel\Appearance and Personalization\Display
However, when plugging in another monitor, the scaling causes the text to be blurry. Turning the scaling off resolves the issue, but leaves the laptop display with a tiny UI.
As far as I can tell there's no way around this issue because Windows 8.1 does not support per display scaling (despite the implication from the unchecked 'Let me choose one scaling level for all my displays' checkbox).
What I am looking for is some way to automatically turn off the scaling when I am plugged into a secondary monitor via the HDMI port and turned back on when just using the laptop display.