Hello, I have a new Retina iMac 5K, and Windows 8.1 installed on Parallels 10. Could You please let me know how to enable scaling modes in video options? I can only enable or disable retina screen.
Make sure the guest OS type is set to Windows 8.1: VM config - General - flat selector just above VM Name. (You can only change OS type when VM is stopped.)
Parallels can run Windows in pixel doubling mode and in native (retina) mode. Retina support check box controls that. When VM is in modality mode, you can resize the window to get any scaling ratio. Font and icons scaling in Windows 8 are controlled by Windows 8 display preferences. What sort of scaling you are looking for?