I went with post and beam rather than conventional stud framing because we haven't yet purchased our doors or windows. In fact our whole design we're pretty much making up as we go along. With the post and beam I can get the frame and roof up and work in doors and windows later. With stud walls I'd be locked in to whatever rough openings I'd made.
Another reason the post and beam approach appeals to me, at least in this situation, is I can keep all the weight directly on the piers. There are 12 piers, so I'll do 12 posts right on top of the piers, and put no strain on the girders.
My posts will be made of tripled 2x6's, so nominally 6x6 [actually 4.5" x 5.5"]. I had the opportunity of discussing my cabin plans with a roof truss engineer at the lumber yard. I did a drawing for him and asked his advice on what dimension wood I should choose. He believed tripled 2x6's would work best, and be actually much stronger than a 6x6 post.
I double up my 2x6's first to create my posts. I use glue and clamps and screws - some nails also. The screws are 2 1/2" drywall, the nails ringshank. Since the floor is dead level, I can basically gang cut all the posts so they are identical in length. I've got eight 13 footers, and four 15 footers for the center ridgeline. The two 15 footers in the middle are actually cut 1 1/2" longer than the others, because they tie directly into the floor without a sole plate.
The reason I don't triple up on the posts is to keep the weight down, while I wrestle with putting them up and bracing them in place. When the frame is interconnected and stable I'll then come in and sister the posts with an additional board so they're full-size.
Putting the first posts up is scary. They're not that heavy, but they are so high . . .! Especially the 15 footer. As long as I don't look up I'm fine. I just focus on getting it plumb right in front of me, then clamp it to the braces I've prepositioned [I've attached diagonal braces to the band joists, so all I've got to do is clamp the post to them when I've got it up. I can then tap it around with a mallet to get it perfectly plumb, and fasten. Screws of course, so I can later remove them].
I also fasten blocks to the center of the floor to give spots for diagonal braces to tie into. Here I've got the first three posts in, securely braced:A closer shot: