I'm not sure of the dimensions beyond the 3/8" hole. I think it's outwardly scoring, and there's a slight ring just outside the hole (5) which is marked by a tool... if your splat is inside that, it's a 5. Then it drops down to 4,3, etc. I heard Ian mention there's a 5.1 you can get for a clean bell strike, i'm not sure if that's done for elimination or if no-one did it on the night.
I heard some mentioning that although the plate stands well, the hole takes a bit of a battering and even hardened steel starts to wear... but then I guess if you're using just the one target week after week, that would be expected.
Probably all that info on the links at the bottom of my waffle, but i'm sure Arfdone will know if not