If I were to bet, that's the 13''.
I'm going with the 13". The keys are the only flat part of it to get a semi accurate measurement from. Assuming it would be the same keyboard as the rMB it'd take me like 2 seconds to find the width out. However googling just brings up a long list of people ranting about the keyboard's size... Someone on a rMB just measure the '0' key and tell me the width!