My shack is 12x24.
6x6 treated corner posts, 4x6 posts @ 8’ on center.
2x10 headers on the sides of the posts (one inside, one out).
A piece of 2x4 is used to fill the gap between the 6x6 corners and 4x6 posts when attaching the header.
All of the construction was done with treated wood, but I really don’t think that was necessary.

4” of concrete was placed at the bottom of each post hole.
Once the structure was complete, the post holes where backfilled with sand.

I went with a ridge beam as opposed to trusses and glad I did.
We have hoods on both the flue and syrup pans, which means 3 stacks total.
Navigating around trusses with 3 stacks would have been a pain.