A co-worker and I both had the same problem after the Windows 10 Fall Creators update (1709). The scaling of Windows 10 desktop icons and windows was huge on a 4K monitor and an iMac 5K. I couldn't change display settings, and restarting Windows didn't have any effect. The simple fix was to maximize the Parallels Desktop window on your Mac and the scaling fixes itself. You can then restore your Parallels Desktop windows to a smaller size and the scaling works. Hopefully this saves someone a few hours.