Ideal BBQ Smoker size for Heater w/default fan configuration?

I currently use my HM with my PK Grill and my cooks are fantastic. I'm thinking about getting a larger smoker and need a few suggestions or my next buy. Any suggestions?

I use an HM on a 1,200 lb. gravity fed charcoal smoker with 5 cooking racks, each 25"x28". There are at least 2 other gents with similar smokers here. You're probably really asking how much fan it's capable of using. Short answer is a LOT.