@Glenn: Please post a screenshot of the problem. And a screenshot of your Windows 10 display settings.
I think it does adjust only it seems as if that's as big the font can be in WinSCP.
You can set different "scale" for each of your monitors.
Since I'm using a second monitor which is a HD 1080 screen, I run WinSCP on that monitor and I'm fine. Only I wish it was possible to have larger fonts for better reading on UHD screens and not-so good eyes anymore like in my case ;)