Don't go shorter than a typical suppressor warranty: 10.5" long.
+
You want to go as short as you can.
=
10.5"
You'll hear arguments about needing the extra velocity of 11, 12, etc. A 10.5 has enough velocity for 75gr T2 TAP, 77gr MK262, 77gr IMI razorcore to be lethal past 150 yards...