Yes, just realised! 
15mm x 6mm was the description of the wound in the scalp. Humes did say that there was a corresponding hole in the skull underneath but he said that 6mm would not be the measure of the smallest diameter. He said that 7mm would be the upper limit of it. 15mm x 7mm in the skull bone would not be out of the ordinary for a bullet entry wound in the skull caused by a 6.5mm bullet strike. It would be described as a keyholed entry wound.