Sorry haven't got the time to look it up for you right now, but you can find it in the docs. It's a setting in one of these .h files either "nrui.h" or "nreal.h"
Probably search for these files in the docs.
Cheers,
FR
Edit: Ah, found it. it's in the nrui.h
gui.viewer.maxWidth and gui.viewer.maxHeight