Officially Jaguar maximum size is 16″. Original size and recommended tires will give you the best ride and speedometer accuracy and of course give the correct rating for such a heavy fast car. Be careful what you fit as a lot of modern 17″/18″ alloys are not really designed for such cars as these. Remember they are very fast and much heavier than what one might think considering how agile they are.
Jaguar knew what they were doing all those years ago 🙂