512 pixels across is the sane and obvious choice for a 9" screen. It's 64 bytes/32 words across, which is a nice and even number. The next step up would be 640 pixels (40 words across), but it's probably not worth the memory tradeoff.
The Apple IIc shipped with the same 9" monochrome monitor and can do 560 pixels across (40 words, 14-pixels per word for historical reasons). It's hard to imagine shrinking the pixels much further on that screen.
The Apple IIgs can draw 640 pixels across in the same width that the Apple II draws 280 pixels. I suppose one could connect an Apple IIc monitor to the IIgs and experiment with monochrome 640x200 graphics to see how the smaller pixels look. (I own both, so maybe I'll try it out).
The result was the GS pixel clock does not align with the NTSC colorburst period. It made artifact color useless, and that was sad because the GS could have done amazing graphics on an ordinary TV.
But, that aside, the overdriving of the color signal meant most users suffered through crappy video, or ran the machine monochrome, or had to buy the Apple monitor.