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According to Amazon the 3605BGD is the big block. Height 36", wide 24", deep 16". I know specs. vary from one supplier to another. I have one 3605 BGD on order. The model you are refering to is a very well used and liked smoker.
That one is the smaller one, I have both. It's a really good smoker, the only problem is if you do ribs you'll have to cut them in half to make them fit. Which is no big deal, I use to just use a rib rack. I'd get it and if you like it see if you can find a deal on the big block. Then you'll have 2 good smokers.