Product review (5 stars): Frankly better made but smaller than expected.
The outside wall frame measures 10 1/8 inches wide X 8 3/4 inches high to the outside edges of the finish frame on the face of the wall.
The inside dimensions OF THE FRAME are 7 1/2 by 5 1/2 but the actual interior room is slightly larger as the frame or faceplate wraps 90 degrees from the wall face toward the interior thereby locking into the section that is within the wall by being inside of it.  It fits snug,y and has serrated grippers.  Therefore, this item will mount just on the wallboard but will not be very tight there.  I found that with 3/8 Sheetrock the 2 in. circular protrusion on the interior piece designed to connect to a drain pipe provided a nice snugger-upper to sandwich the wallboard against the back of the frame but you will want something more substantial to secure it.  I ran screws through the Sheetrock into 2x3 framing verticals on either side.  The screws are hidden by the face frame anyway.  The securing straps it comes with are useless, however it is quite sturdy in its own right.

It looks too small before you install it, but once it is in place and connected there is plenty of room to turn a regular one lever OR two standard 1/4 turn faucets on and off easily.

I did not want the hoses IN FRONT of the wall pushing the machines out FROM the wall so I built a false half-wall that only comes down as far as the backs of the machines.  I then dropped the hoses through the drain hole where they wrap behind the bottom plate frame and back to the washer and the actual drain line is mounted in back of the wall, which is easily accessible from the side access door I made to allow storage of vacuum cleaner and ironing board.