A 6 year old patient, who was admitted for a dog bite on the face, unexpectedly developed anaphylaxis to a test dose of Augmentin. He developed stridor which needs immediate intubation. You are the only available doctor at the time. You have a No.3 curved blade laryngoscope ready and you have asked for an Endotracheal Tube (ET) to the nurse. What should be the appropriate internal diameter of the tube?