Depends on how long you let them grow. I've seen some around 14lbs live but I believe they can get bigger. They are BIG chickens if you let them grow out all the way.
Having said that most people don't because there can be serious mortality issues with that. But if you are careful raising them to those sizes you can be successful.
The largest I've had was a 13 week old rooster who was 10.5 lbs dressed. I restrict food and give them access to a large yard -- the fellow was still pretty spry and could have gone longer. I've read about people getting 14 lbs dressed birds.