I use 14-gauge, 2" square tubing on EVERY aspect of my builds. This means I get a better price on the steel (buying it in pallet-sized quantities). I haul it with a 30-foot farm trailer.
I fabricate with both a Lincoln "cracker box" stick welder using 1061 or 6110 rods -- I'm kinda dyslexic at times -- when fabricating everything EXCEPT when I have to weld overhead. Then I use a Hobart .030 flux-core MIG. I'm a year short of 70 and holding a heavy stinger above my head, not to mention the shower of molten sparks drives me to use the MIG whenever I can. I turn the power up to achieve better penetration.
All walls, both interior and exterior are 2". If I want insulation, I use sprayed-on, closed-cell foam (wicked expensive, by the way). If I have an interior wall on a plan, I bury the support posts just as if it were a perimeter vertical support. My stuff is built like a brick shit house, my having been 'bullseyed' by an EF4 tornado about 5 years back. Since then, I've become a big fan of concrete and steel construction.
Exterior walls and interior walls (if covered) are done so with "R" panel, screwed to the steel framework. All electricity is pre-planned using EMT (metal conduit). It makes pulling new wire (should the case arise) a breeze. It looks a bit "industrial" but like I said -- brick shit house is my goal.
I fabricate my own trusses and place them atop opposing 10-foot-centered vertical framing members. I do not "pre-load" the bottom chords (they are not engineered) but I do "gusset" triangle corners with welded 11-gauge steel plate. "Rafters" are 2" square tubing. I used to use purlins, but I've found 2" square tubing, properly welded in place is stronger and easier to hang by myself. Square tubing tends to be more stable (bending forces) on longer runs than perlin. If I'm doing an attic space, I lay undulated galvanized sheet metal down, screw it to the attic side of the ceiling structure and pour in floatsome, which is air-entrained concrete. This gives me a ceiling, which will support massive loads of crap and allows me a second roof, should Mother Nature decide to rip mine off. Winds in North Texas are brutal and Mother Nature is, well, nothing short of a female dog!
Hope this helps. As soon as I have something in "frame" I will snap some pictures and send them along.