I was trying to figure out this same thing. What commanders do I need for a 2000 man (for example) infantry brigade to avoid the efficiency penalty? It turned out to be pretty simple:
For 2000 inf, you need 45 command (the brigade stat right at the top). I hadn't checked higher sizes, but taking a few sizes up to 2000, it looked like the command required is around (size - 500) / 33.
Brigade command is around 0.1 * brigade_cmd_rank + 0.05 * division_cmd_rank
(The corps commander does not matter for command.)
I valued the commander ranks using the following XP ranges. (You can find where they are in the range by hovering the mouse over their experience bar.)
0-99 captain
100-199 major
200-299 lt. col.
300-399 col.
400-499 brig. gen.
500-599 maj. gen.
600-699 lt. gen
So, the upshot is a brigadier general with 50 XP toward his next rank (450 XP = 45 command) could command 2000 inf essentially on his own, regardless of the division commander. A major with 50 XP toward his next rank (150 XP = 15 command) would need his division commander to be a Lt. general (600+ XP) to command the same number of men without the penalty.
Of course this all may have changed since I measured it I think some time in late December. Also I was only looking at CSA, so I guess the rules could be different for Union?