Expanding to 29 or 30 inches would drop it below Retina, but not terribly so.
Still, I would guess Apple would go 5.5K at 30" (5856x3132).
The definition of "Retina" varies because the theories of what constitutes "Retina" vary. Remember, Jobs initially stated Retina was about 300 ppi, but that referred to a phone viewed at 12". The effective number would be lower for a laptop, and lower still for a desktop because of the distance to the screen.
Interestingly, if you use
this website's mathematical definition of Retina, a 30" 5K 5120x2880 screen is Retina at 18" or further viewing distance, despite being "only" 196 ppi. Furthermore, a 32" 5K 5120x2880 screen is Retina at 19", at 184 ppi. At 29", it's Retina at 17", at 203 ppi.
It should be noted that OSHA states the minimum monitor viewing distance should be 20", so any of those would be considered Retina at that distance. Thus, a 29-30" 5K screen would be great. Apple's old 30" Cinema Display was actually 2.5K 2560x1600 and 101 ppi, so a pixel-quadrupled 5K 5120x3200 version would be 201 ppi, and that more than meets their definition of Retina.
But as mentioned, they won't do it, because they've standardized at 218 ppi. All of their all-in-one desktops and monitors are now 218 ppi.