The output size is defined by the panorama size.
When recording to HDD or broadcasting RTMP. You get an option to scale down the output for each encoder.
When using SDI output, the panorama width must match the SDI output frame width.
Eg. : If you want to use 3840x2160 output a Decklink card, the panorama size must be set to the same width: 3840x1920
If the panorama size is not compatible, a warning will pop up when you try to activate the SDI output.
The panorama size can be set from the configuration panel: