There really is no standard good enclosure size for any type of tortoise, only opinions which vary greatly. I agree with yourlocalpoet, the bigger the better. A leopard that is 8 inches long is a pretty good size, so just to throw a number out there I would say a minimum of 10 feet by 10 feet.
Welcome. Wow, 8" at 3 years - sounds like you have a true pardalis there.
A minimum of 10' x 10' because it is only going to grow. I have a 10" and she wanders all over the back yard every day. I built a 6' x 6' enclosure in the garage for winter and it was definitely too small for her. Luckily our winters are only about 2 months long here.