cold smoke or hot smoke? the bigger they are the more fuel it takes to run 1 , its amazing what you can pack onto shelfs when you need to , having a good drain system and a way to wash said system out is as important as the wood you choose imo, keeping it clean and bug and rodent free can be a challenge imo. 2' deep 4' wide 8' tall can hold a whole lot of any thing you can think of to smoke, making racks and ways to adjust to any size would play a important part of a long term smoker for hot smoking, the cold smoking traditionally have shelfs /tables with meat salted/curing and are left unheated after the smoking is over so more room is needed, I think my grandfathers was somewhere around a 12'x12' thinking back. The last 1 I remember was my uncle Franks and it was a 10'x10' and he mostly cured pork ,grandpa had 13 kids, took a lot of everything to keep them fed lol