• 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.

    Also remember that SwitchResX isn't designed to enable the HiDPI capability on ay Mac.
    HiDPI should be enabled separately, and SwitchResX will manage these resolutions when HiDPI mode is enabled.

    Apple Silicon Macs:
    Please also see the specific FAQ for HiDPI on Apple Silicon - in particular, you should use macOS Monterey 12.4 at least

    Intel Macs:
    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.