how big is this watch, i.e. how many millimeters?

52mm diameter Case Thickness 17 millimeters

I know the dimensions, but does it appear "too big" for an 11-year old boy's wrist? He's athletic and we want to get him a watch for his birthday.

The G7900, while fairly large, has wings under the strap that make it fit nicely on smaller wrists. This means that it will likely fit better on a smaller wrist than similarly-sized G-Shocks that don't have the wings under the strap.I bought a G7900A-4 for my nephew recently. I was so impressed with the styling and the bright red color that I ended up buying another one for myself. Of all the watches I have -- and I have more than I need -- this one attracts the most attention. People who don't even notice my other watches definitely notice this one. The bright red color is hard to ignore.I think an 11-year old would be thrilled to get this watch, and will likely be the envy of his friends.

is there a smaller size?

We at us legend inc would like to thank you for your email.We believe the Casio g shock G7900 series comes one size only (large)We would like to thank you for shopping on Amazon.Us legend inc customer service

Is this a legit casio g-shock? or is it like a replica? i gotta make sure i'm not buying the replica one, i don't want to make that mistake again.

There are certain specific markings on a watch manufactured by Casio. However to be one hundred percent sure that you did purchase a Casio product you would have to mail it in to Casio’s factory service center in order to confirm that the watch was manufactured by Casio.