Ir depends somewhat on the varietal of Hydrangea you have. Some, such as Annabelle produce suckers and are even more difficult to control size-wise.
But, having grown Bonsai for a number of years, I tried this on a largish Hydrangea at the entrance to our garden; In the spring, before the plant shows signs of leafing out, sharpen you garden spade ( a square bladed one works best) with a file. Cut about 3 to 4 inches away from the main root ball all the way around and discard. This, along with some moderate pruning will control the size. Bonsai are taken from their pots about once every two years, the roots combed out and cut away randomly to control the plant.
Best of luck!