I just looked at those pics again. If by "porch" you're refering to the porch under the roof, there's next to no load there at all. If all you have is that little framing & steel roof, nothign more above it, you could damn near hold that roof up temporarily by hand.
The pic's will best describe the current status the two posts (pink and new cedar are braces for this part of the porch, although this particular post doesn't seem to be supporting any weight it probably helped hold it down in the sometimes violent updrafts adn gusts of wind).
I"m just doing one post right now. using a 24" long sonotube and 8" wide.
the old "feet" were 8" wide and i've no idea how deep. they were cracked/crumbled.