Post holing can kill you!
Go big as you can!
Today, used old school (SEAL issued ATLAS 1033...am NOT a SEAL). Newer models are 1035 or 1040, I think. Good for pretty heavy loads.
TUBBS brand is durable and quality.
Your weight + gear weight, angle of slope and depth and condition of snow determine the size needed.
Do NOT buy discount, cheap or light duty stuff!