What was the maximum screen resolution in Macs based on 68K CPU's?
If I recall correctly (happy to be corrected) the most powerful 68K Mac that was actually released commercially was the Macintosh Quadra 840AV. It was launched in July 1993 and discontinued in July 1994 and actually overlapped the period during which Apple transitioned to PowerPC processors.

Now, depending on what you're actually asking, the thing about this model is that it had the ability to support a second display because users could expand it by inserting a NuBus video card. Depending on the card and display being used, the amount of VRAM installed, and the numbers of colours being output, one could achieve a maximum resolution of 1,152 by 870 pixels.

For example, the Macintosh Display Card 24AC and the Radius PrecisionColor Pro 24AC displayed this resolution (1,152 by 870) at 75Hz. However, the 1,024 by 768 resolution used by most was supported at both 60Hz and 75Hz.

Finally, just as an aside, I refer to the following comment in your question:


  There are reports of Mac OS 9 going up to 1920 x 1200, but that is, of course, running on a PowerPC CPU.


I still have PowerPC based Macs that can run macOS 9 natively and display a resolution of 1,920 by 1,200 pixels, but this has nothing to do with the PowerPC CPU per se. Instead the displays are driven by the various NVIDIA GeForce graphics cards you could have it configured with.