It is not up to your groomer how short your dog has to be shaved.
The matting determines how short your dogs coat has to go.
Your groomer has to use a blade that will safely clip between the mat and your dogs skin.
If the matting is not too close to the skin, the groomer may be able to use a longer blade safely.
If the matting is tight and close to the skin, the groomer will have to use a very short blade.