The reason I take 3 man units is it’s easier to get the 9” wholly within khorne buff. I would absolutely take 6 man units instead to get more of a buff outta the doombull, but honestly because Gorshrak debuffs the ENEMY unit so that MY units get +1 to hit - MSU is fine. Further, you tend to put activate your opponent and it lessens the chance that an entire unit goes poof.
Btw the argument for a bigger unit is that you can get a much higher usage out of the doombull buff, and throw more attacks - but meh.
Gorshrak in my opinion is a must take now for BoC - almost on the same level as Belakor. His spell does D3 mortal wounds (meh) but any of your units attacking the affected unit now get +1 to hit. Oh did your opponent bring 30 super dudes... enjoy everything that charged them getting +1 to hit!