You have to follow the instruction carefully :
Well, switching the desktop resolution to 720p59 when you play a 720p59 MKV, then back to the default resolution 1080p59 after finishing the playback is easy as well, assuming you already know madVR's framerate token.
1. In madVR settings > devices > "your display" > display modes > "list all display modes madVR may switch to:", add "720p59" (without quotes).
2. Add "720p59" (without quotes) somewhere in the name of a 720p59 mkv file.
You can also use "720p60".