The Frankenbrew answer
I forgot most of the heat transfer I learned in college but if you want to size it appropriately, you would need the temperature of the surrounding air, the temperature that you plan on holding the beer, the heat transfer coefficient of the stainless steel and the copper, the square inches of the fermenter, the square inches of the copper that's touching the fermenter, the square inches of the copper that's exposed to air, etc. Ah yes, engineering school was fun . That being said, I'd throw the heat transfer book to the side and just use the frankenbrew video as a reference. In the video, it looks like Tom used about 10 bands of 1/2'' copper to wrap around a 7bbl grundy and that was able to both hold the temperature at fermentation as well as drop the beer to conditioning temperatures. So assuming a 36'' diameter grundy, that would be about 1130 inches of copper tubing he used (pi x 36'' x 10) for 7bbls. Also keep in mind that his was an un-insulated tank so by insulating, you might be able to go much shorter. Sounds like the fermenter you're using is around a 4bbl fermenter so somewhere around 500 feet would be my first start and even that may be overkill.
I'd say the diameter of the coil isn't as important as how much of the copper is actually touching the tank. If you go with larger copper tubing, just make sure you tap it down to be as flat as possible against the fermenter. I'd also try to get at least 1 coil towards the bottom of the cone since a lot of the "yeast heat" is going to be down there. Hope this helps some.
Kaskaskia Brewing Company