So is the max of VGA 2560 x 1600?
The max is as good as your RAMDAC, output drivers, connector, cable, connector, inputs and surrounding environment are. In other words - not that much. All kinds of analog nasties caused by pass-band ripples, signal reflections and noise.
And if you feed VGA to a digital display, it has to be sampled to digital again .. lovely.
I still want extra VGA input on any monitor/TV device around me - plenty of retro devices to connect, and easiest to drive.