Is there a surgical procedure that can shrink a really large forehead?

Yes bone and skin: Yes but it depends on whether you are refereeing to excess skin and a "high" forehead or to excessive prominence of the forehead (frontal bone). In then former, an incision just in front of the hairline can be used to excise the forehead skin and advance the hairline downwards to reduce the vertical length of the forehead. In the latter, craniofacial surgery with boney contouring may be required.