Of course, parishes vary considerably in size - I believe the Beddgelert parish boundary is around 30 miles in length, whereas one in a city centre might be about a mile (and far less fun to run round!)
My Brother-In-Law was vicar of Bleanau FFestiniog and other parishes. His patch included most of the Paddy south of Snowdon and the top half of the Rhinogs. A huge area.