Have never built a smoker before and going to make my first attempt. Going to use 20" pipe 5' long, opting for the reverse flow type. Was going to make the fire box sqaure(rectangle)18x24. I just dont know how large to make the firebox opening. I also think im going to use 4" pipe for the exhaust, think im on the right track, but could be wrong. You know how us newbies are .