These North American turtles, that are known as Red-Eared Terrapins in the United Kingdom, can grow anywhere between 25 to 30 centimeters for the female and a smaller 20 to 25 centimeters for the male.
A really big female red-eared slider can have a carapace (A carapace is the top of the shell) length of almost a foot. Males are a little smaller at around ten inches. Those are the upper ends of the growth spectrum for the species though. All red-eared sliders sold in the United States must be over four inches.