This ammonite fragment is one of the biggest in my collection; it's heavily worn after being tumbled around in shingle so it's hard to decide whether it's a Hilderocas bifrons or Arnioceras semicostatum.
Since it was blowing a 60mph gale, I decided that today would be a good day to take it to the beach with a pencil. And an 8 inch adjustable spanner for scale. The tape measure would have blown away, you see.
My very rough primary-school standard drawing around things suggests that the ammonite was 7-8 inches (14-16 cm) across. If there are fragments on the beach, there should be whole specimens in the cliffs waiting to be found.
If it's Arnioceras it's Lower Liassic, Sinemurian, Semicostatum zone.
If it's Hildoceras it's Upper Liassic, Toarcian, Bifrons zone.