A toy that long won't do any harm a long as it's not sharp and doesn't have any rough edges. A toy that wide would be uncomfortable and might cause some tearing without caution and preparation.
Using lube helps objects go in with ease since the anus doesn't offer any natural lubrication. It's not strictly necessary but helps with comfort, as well as keeps the object from dragging and catching skin which could cause irritation or slight tears in the skin.