The key is heat and movement of the beans. You can't let them sit in the pan, so this is where the stamina comes in. If you put too many beans in the pan you won't be able to keep them all moving properly. This is not the method for roasting large amounts of coffee for later. Typically when I use a cast iron pan, I'm roasting just enough for one pour over the next day, about 16-20 ounces of coffee to drink to be shared with the wife. I'll use about a quarter cup of green coffee beans, maybe a little more.
You'll also get some chaff, so make sure you have good ventilation. If you do it right, there won't be enough smoke to worry about, unless you're going for a dark roast on purpose. If that's the case, there's going to be lots of smoke.
-Mike