If you have the standard tyres 165/80/15 a 13" steering wheel could work because you don't need the steering force you would when driving a roadster due to narrow tyres and lightweight engine. Nevertheless you should give it a try before buying it.
I have a Momo Prototipo fitted on my 4/4 which is about 13.78 inches and I'm quite happy with it. And yes, the steering wheel should on one hand allow to handle it having some distance to the dashboard and on the other side let it not come too close to you because the relation between pedal distance and steering wheel distance in a Morgan is on the edge where you need long legs and short arms.
