i think an 8'-8' flat down would be long enough for a backyard setup. sound fun man, post a pic when your done, or message me, i wanna see it.
btw, i had a welding class a couple years ago. it was madd fun, and for my final project i got to make an abstract sculpture of a dog =P
wish the school im in now offered a welding class; id take any day hands-down to an art class.
note: everything underlined is sarcasm