Our Barn Style Garage walls on first floor are often framed out with Rough-Cut Oak lumber. We notch weight-carrying posts on top and place heavy duty LVL wall headers in those notches. This way, all of roof and loft weight is not held just by nails. Clear span beam to support loft is tripple 1.75"x16" or 18" (depending on span). Floor joists are normally 2x10 SPF with 3/4" T&G plywood deck. Roof rafters are either 2x10 or 2x8 with extra knee-wall supports.
This is not your typical pole barn and is built extra heavy to look good and to last long.