If you set multiple comma-separated screen resolutions, a connected RealVNC Viewer user can cycle between them using the command line utility xrandr, or the virtual desktop's Display/Monitor settings app. You can specify one or more screen resolutions (geometries) for a virtual desktop using the RealVNC Server RandR parameter.
The method to do this depends on whether you are have enabled SystemXorg mode or are using the older, legacy Xvnc. When running RealVNC Server in Virtual Mode to access a virtual desktop, you can freely change the resolution of the remote virtual desktop as you need to.