I'm building a small timberframe(16'x24')that is 2 storeys, 6/12 pitch roof, 8' between posts. I have some(limited) experience timber framing(1 class and 1 small building). I have used formulas in Benson's book to determine the beam size needed for the first floor bent girts and 1st floor connecting girts to be 8x13. Does this seem excessively large. I am building in Southeast Alaska which has 50 psf snowload. Thanks for any advice/insight.

Without knowing the type of wood you want to use, and the spacing it is hard to answer your question.

I'll be using select spruce, bents are spaced 8' apart, 1st floor girt with center post at 8' also.