It looks that all the pain with WinBox and high-dpi-screens is gone with the latest Update of Windows-10.
Microsoft introduced a new scaling technique that will make WinBox look good at other than 96dpi/100% display scaling settings.
Have a look at the images below.
1) right click two times on the Winbox in your Taskbar, then left click "Properties"
2) The Properties dialogue appears. Select "Compatibility". Choose the scaling option "System (Enhanced)". Click Ok.
3) Start your WinBox. Et voilá! It is scaled properly.