Obviously if you prune it down to the ground you won't get any flowers, however on last years stems there will be a number of shoots running up the stem. I've read in a book specifically on hydrangea's and it says the further down the stem you prune or the less shoots you leave on the stem the larger your flower heads will be by late summer, however it did warn that these stems with large flowers have a tendancy to droop due to the weight of the flower.

I haven't tried this myself so can't comment, but may try it next spring on one stem to see the difference in flower size