I just got finished planning and having my second stand built.
I can speak to the heavier end of the spectrum as for strength. Maybe some others can chime in concerning where things start to get dicey.
I had my last one done out of 1.5" x 0.125" thick angle iron. There was absolutely no worry about strength with this stuff. On my second stand, I was debating 1.5" box tubing versus 2" box tubing. Strength was never a concern between either of these, but I was interested in what others had done. On one of the forums, I read a comment that summed it up. When in doubt, build it stout. Now I don't know if doubt was what I was experiencing, but I'm a sucker for a good rhyme. And 2" box tubing is what I went with (for the frame). 1.5" box tubing for the kettle supports was plenty.
But if you don't need to use tubing for some reason, i.e., gas beam, the thicker angle iron will be extremely robust.