roushana, you need to be more specific with your terminology. I think you are referring to a GEC. If that is the case, it is sized per your service entrance conductors listed in NEC table 250.66. It can be bare, insulated, stranded, or solid.
If you are referring to an EGC, it can be any of the methods listed in 250.118, or if a copper conductor is used, it is sized per 250.122.
So what are you talking about? I hear the term green wire ground used in the telecom (or ACEG)sector a lot to reference an EGC, but it is not proper terminology.
What is the application? Is it a box pigtail; GEC; Etc.
Please provide more details if you require a less generic, more helpful and/or safer answer.
[Edit add: Article 680 comes in to play if you are working with a pool/spa/hot-tub circuit or feeder].
[Edit add #2: Strike that. Subject says "Earth Ground". Is it just me or are questions harder to answer when they are too brief and the gist of the question is not re-stated in the body of the post? I find it helpful if the complete question is asked in the body of the post, not fragmented or solely contained into the subject line].
PS: If your reply is it's just me, there's no need to state that publicly