Outside" 6" wide X 61/4" high.
Only if you purchase the optional support bracket that is not carried in the store, otherwise there is nothing to attach this to inside of the vanity.
When you cut the wall and you find some structure (wood) to fasten with provided screws, it will be very easy. I didn't have any structure I could fasten. I bought Moen 588 Donner Clamp from Amazon for less than two dollars with free shipping. Moen said this clamp is for this holder in their web. But I needed make holes (actually they were slots) larger. I had to buy two long stainless bolts and Nylocks. The bolts needed to be very long. Thread bolts through the front of holder holes, and clamp holes, and add Nylock until the end of bolts come through the nuts. In this time, there are lot of spacs between holder and clamp. You twist and turn the clamp till it goes behind the wall. Then, straighten the holder as your specification. Tighten bolts, pulling the holder toward you so that bolts goes in until the clamp and the holder sandwich the wall tight. I wish Moen give better instruction.