We recommend using 6x6 for deck support posts. 6x6 has about 2.5 times as much surface area as a 4x4. If the 6x6 is above 12' high bracing is required to stabilize bending moments.
Compression strength (wet to dry)
Douglas Fir is 1300-1700 psi (lbs per square inch)
Pine 960 -1200 psi