I did it with a friend of mine, and he's 8 inches, and it worked. The thing is though, you won't have enough of the material if you Don't use the vibrator that is part of the kit. The vibrator takes up quite a bit of the space, therefore, not as much mold material is needed.
You have to buy 2 of the kits if you have a cock your size, and don't want to use the vibrator that's in the kit.
Oh, and getting a refund ??? That would be a hoot to see you try to get a refund at the sex shop !! LOL "My cock is too big for it"......I can see them asking you to whip it out and prove it.