I built one last fall. The smoke chamber is 2 55 gallon barrels and the fire box is a single barrel. The sucker is so large it has its own trailer that we haul it around in.
Our next project is to get something around a 10 ft propane tank and put it onto the back of an old farm truck! Should be pretty cool.
Oh by the way. When getting barrels if you go that route, make sure they are food safe/find out what was in them. We got lucky that there is a bakery in town and they use barrels for jelly. Want to make sure they didnt have oil or anything like that. You can burn it out, but there are so many small pores in the steal that you will never get it all. Good luck!