Since he'll be using the board for light fluffy DEEP powder a 161 will work. He can probably go even longer depending on how nimble he wants it to be vs. floaty. The Proto comparison doesn't really apply as he's looking for a dedicated pow stick and hight really has no impact on board size.
+1. I am a little taller (5'9") but lighter (145-155lbs) than the OP and ride similar sized boards (155-159) for the day-to-day stuff. But for big powder days in Niseko I pull out the 163 Sick Stick.