When you buy acetate strips to do chocolate decorations around cakes should they be 3 or 4 inches? I would assume 4 but what I mostly see for sale is 3. Thank you!

I see a variety of sizes 2.5 - 4 inches for strips and 1 - 4 inches for rolls:
http://www.bakedeco.com/detail.asp?id=21589&keyword=acetate

I'd purchase 4" to go on 4" tall cakes or 3" if my cakes were only 3" tall.

And where do you buy the acetate? What department?

I buy acetate at the art supply store and they cut and sized it for me.