Jersey Giants, especially hatchery Jersey Giants can be the actual same-size as other breeds. The only thing that you MIGHT be able to tell the difference with is if they get larger than the other birds "eventually." Some have yellow on the bottoms of their feet, and others have pink which makes them look identical to hatchery Black Australorps.
The "green" should get darker as they age and will make their entire leg look a "slate" color, or it might stay a greenish glaze.
The "green" should get darker as they age and will make their entire leg look a "slate" color, or it might stay a greenish glaze.