Hi AJ I was thinking more about your building, and should say that your timber size will be very much determined by what you need to best accommodate the joinery. Without knowing how you plan to layout the joints, I can guess that you may want to size the timbers the larger size for joinery purposes. If you are planning to dovetail your floor joists the larger timber would probably be good. If you get some drawings together it would be great to see them.
