The old limit for practical corps size was 25,000 to 30,000 and was defined by march duration and pass time. Think of men marching in column. Width is 5 men on one road, average spacing is 1.5m. Total column length for 30,000 men:
6000 * 1,5m = 9 km. Pass time at 4 km/h: 2.25 hours.

Now keep in mind all those frictions and it becomes visible why larger formations were unwieldy.

Another reason for corps size limitation was overview over a battlefield (few km width can be controlled by a single commander with telescope, flag signals and courier horsemen).

None of this is relevant today.


(Btw, a pre-radio army or corps commander had much more direct subordinates than three!).