I know this is an old thread but i know how to solve your problem.
If you want to detect highest resolution that is users monitor able to use, just simply use node “Get supported Fullscreen Resolutions” which returns array of all possible resolutions that users monitor can do, and the last one in this array, is the highest one.
So jsut set the resolution to the last resolution in that array.
WARNING: This BP node returns resolutions in format without “x” in between. For example 1920x1080 will return as 1920108. So you need to put “x” in between the numbers if you want to execute console command via BP, otherwise the command will not work.
Have a nice day!