We recommend using 6x6 pressure treated posts for deck supports. In the past 4x4’s were commonly used for support posts but 6x6’s are preferable because they are much stronger, more stable and offer a larger surface area for supporting beams and contacting the ground. We have seen a 4x4’s bow as much as 6” causing the entire deck to sag 1”. Yikes! Many building departments won't allow you to use 4x4's if you are building more than 60" off the ground. If the deck is over 12' high we strongly recommend installing bracing to stabilize bending moments.
Compression strength for 6x6's (wet to dry)
Douglas Fir: 1300-1700 psi (lbs per square inch)
Pine 960 - 1200 psi
Buried support posts or installing posts in direct contact with the ground require using .60 RET ground contact rated pressure treated wood to prevent rot and decay. You want to purchase support posts that are as straight as possible because long wood posts have a tendency to twist and bow. 6x6 rough sawn cedar posts can also be used for supports but are very expensive and should never be buried in the ground. Wood support posts are prone to checking and cracking because they tend to dry out unevenly because of their thickness. The higher the moisture content the more severely the posts will crack.