Ummmmm, I don't no what you mean by terrapin. A terrapin is a turtle that lives within fresh or brackish water. So the only things not terrapins are tortoises, box turtles, and sea turtles. Do you mingy a diamondback terrapin? If so, males are about 5" and females about 7.5". Hope this helps.
Answers: "Terrapin" is a very broad term, that is taken wrongly by tons people who use it to refer to ANY aquatic turtle. If you do an image dig out on "terrapin" you'll see that people use it in reference to everything from red eared sliders to deep-sea turtles to snapping turtles... NONE of which are true terrapins.