An extra-toes cat isn't a breed. It's any cat born with polydactyly, a harmless inherited variation that adds toes, most often on the front paws. Some have a mitten-like thumb. The trait is dominant, so a cat with one affected parent may have it too. The name "Hemingway cat" comes from the writer's six-toed cat in Key West, and it now covers polydactyls of any kind.
Most extra-toed cats in shelters are domestic shorthairs or longhairs, so size, coat and personality vary a lot. Adult weights usually fall between 6 and 15 pounds, and lifespans of 12 to 18 years are common. Expect moderate shedding and a bit of extra attention to claw care, since the extra nails don't always wear down on their own.