Ugh, well, hydrangeas were not "designed" to be divided but there are posts in the forum where people describe doing this with a spade or hatchet or a saw. It probably would be best to do this to a large/old specimen & do it once the plant has gone dormant in late Fall or during the winter months.

Would cutting the stems at the edges down not help "shrink" them? Propagation by cuttings would be more reliable. The instruments used to cut actually destroy sections of the base/crown. Plus you need to end up with a section that has a few good stems and enough roots in order to build a new plant.

Hydrangeas start very easy by cuttings or by layering.
Mike