This is already supported on Retina screens and we'll update our FreeRDP plugin to include support for setting the scale factor even on non-Retina screens in the next update.
This is now available in the latest beta (188.8.131.52).
As usual, you can get the beta here: https://www.royalapplications.com/go/kb-ts-mac-downloadbeta
Microsoft has been supporting setting the DPI of a Windows Remote Session for the last couple of years, and other apps like Microsofts own Remote Desktop Client and Jump Desktop has implemented this feature.
I really miss this setting from Royal TSX. This is supported on the Windows-version of Royal TS, but still not on the Mac.
I hope you can implement this feature in the near future.
Example screenshot from Jump Desktop, where this feature is called "Desktop Scale":