Here is a Devonian coral fragment I found a while back. I decided to try something a little different to see what size it would have been. The Erwin Stumm Devonian and Silurian Corals of the Falls of the Ohio book lists this coral as having a maximum size of 15 cm in diameter.
I believe this coral was from the Jeffersonville limestone.
It looks to me that the coral was probably 11-13 cm in diameter.