Bski I think I would consider going 36 ft wide on the bottom and 20 ft wide on the top. I would incorporate 2 lam beams running long ways under the upper level walls. Then your floor trusses only need to span 20 ft. The outer span can be 2x4 or 2x6 or left open because it is only a ceiling. I know your thinking of diy build but lam beams are fairly cheap and can be erected with a crane truck on site for nomally under 500$ depending on how far they have to drive to the job site. The beams would be flush with the ceiling if you use joist hangers. You might also consider the "X" open I beams it makes utilites much easier to plumb. Just my 2 cents.
