Goodlooking car Paul!
I'm not really sure but I'm PRETTY sure that it's the 8 1/2" version you'd want.
The Porsche insignia was almost a neverending saga of change, even within the 356 family of cars.
If you go to a site with a lot of photos of original cars you can find ways to compare the sizes of insignia to the sizes of known objects - a US license plate, for example is such a known object. Find a photo with both the insignia and a plate and you'll have your answer.