How can I define a new HiDPI resolution of ... ?
You can try to define any HiDPI resolution by creating a new scaled resolution with a doubled size.
For example, if you want to try 1920x1080 as a HiDPI, define a custom scaled resolution of 3840x2160.
Please note that not
all video drivers and all macOS versions will handle all resolutions.
There are drivers that do not work with scaled resolutions bigger than native resolutions. Also, there are drivers that can manage resolutions no bigger than 4095 pixels wide. Some other drivers can manage up to 8191 pixels wide.
Finally, if no HiDPI resolution is getting activated at all, check that the Retina mode is first active on your Mac, by typing:
sudo defaults write /Library/Preferences/com.apple.windowserver.plist DisplayResolutionEnabled -bool true
in a Terminal window.