Whilst I don't have knowledge of how to do a headless change of the Euclid VNC resolution specifically, some Linux users change resolution in headless mode using a system called xrandr. I believe the VNC software has to support xandr for it to work though - RealVNC and TeamViewer do, but I do not know about the VNC solution Euclid uses.
I'm sure one of the Euclid team will soon give an official response to your question.
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