Is the keyboard on the 15" Mac Book Pro with Retina display different than others in size?
No need to worry - at present, the only MacBook with a physically different keyboard size (in the US) than the rest is the 11 inch Air.

The small Air has a full sized "main keyboard" but the top row of function keys are slightly smaller than the top row of function keys on the rest of the lineup. I would presume that the same holds for international key layouts when they have different enter keys or additional physical keys, but don't know that fact for sure.

The cover you mentioned will fit the new retina MacBooks - both 13" and 15" - as long as the hardware is a US model.