I don't think that can be answered as asked, a post of some specific dimension interchangeable for any situation of load and effective length. The effective length of the column versus the stiffness of the species and grade is a big variable that makes a simple blanket substitution impossible I think.

I've tried to work a stripped down example below. This would be a pin/pin connected column with only an axial load, a simple post. I worked the problem with a 6x8 hemfir post 10 feet long and maxed it out at about 22,000 lbs. You can back out and re-enter the desired dimensions or species to find a substitution and get a feel for the variables.

http://windyhilllogworks.com/Calcs/simplecolumn.htm

Hopefully this will help explain why a simple answer escapes me.