Replying to Metal rail in shop class help

I am gonna make a rail for my "final project" in shop class. We obviously have a welder, so it's gonna be metal. I was thinking a down flat down, to make it a little more challenging then just a flat bar, so i get a better grade. What should the dimensions be? it can't be a fucking huge rail, so i was thinking like 10-12 feet. either 4-2-4 (feet) or 4-4-4. Do those sound good? and how high up should i start and end it, and how long should the supports on the bottom be so that i can spin on and stuff without it tipping over? I know, build a jib, but it's kinda dead.
And i did search bar, so don't even say that
Thanks