[We spoke by telephone since we know each other from another life.]
I'm not sure what the limit is, but users should be aware that OpenGL rendering is not meant for huge images. I suspect it is related to the number of polygons in the image as well as the ultimate window size.
There was a time when making the OpenGL window too large merely resulted in jerky navigation, but with Archicad 10, the problems include the mysterious lines we are calling the
Spiderwebs
Of
Disaster (SOD).
Usually they are confined to the OpenGL rendering window, but we have now learned that they can extend to other display areas, too, as a minor comeuppance for having two huge displays.
Dwight Atkinson