What Peter said but I will add a slight twist.
Too small a roaster and you spend your days slaving over the thing. Now a bit bigger than needed roaster, yes it's a bit of capital tied up but roasters typically hold their value pretty well if you take care of them, BUT.... you are a nobody now (no offense) once your business gets going and people know you, and your product, you become a somebody, and your sales go way up, so you grow into that big roaster you got, instead of having to sell a used roaster and then buy yet another bigger one.
Just my.02. and also what peter said, the roolz and bullshit of running a business, it may be good to get in with someone who can do all that for you, .. even if you do get very big, you will have to hire someone to do all that stuff anyways, maybe discuss this with them and buyout options etc before you get all set up. Not only health department but once you start roasting over 5lbs a shot or whatever it is, now you are causing 'pollution' and need a capture system for the smells / vapors bla bla type stuff. Having someone who knows this kind of garbage beforehand helps you wade through the red tape to finally get going.
Aaron