Anytime you want to try some timber framing just give me a call. While you were watching that barn go up, I was working on taking one down. This was an old "Onion barn" in Danvers, Mass. that will be re-erected in Ipswich, Mass at the Cuvilly Arts Center in July. The cranes sure do make the work go faster. It is quite likely that they used a gin pole to put this up originally.
As for post widths, it often comes down to a combination of strength and aesthetics. Oak is quite stron but also shrinks and checks a lot when drying (when compared to white pine).
Nice looking project - I think I saw some pictures of it in the Nashua Telegraph.