Just do not prune it. There is usually no reason to prune provided the shrub has been planted in an area where it can attain his estimated size at maturity that the plant label specifies.

Hydrangeas are not heavy feeders like roses are so they do not respond with huge growth spurts like roses do. They just take a sip of minerals and that is it. One general purpose slow-release application usually does the trick for the whole year, with minor weak fertilizers like coffee grounds and liquid seaweed (or liquid fish). If you are in a hurry to have a larger hydrangea, it may be best to just pay extra and get a larger shrub to begin.

I do not mean to sound that they grow as slow as camellias though. Just want to point out that they do not respond like roses. But depending on how much vigor the shrub has and how well of a root system it has, it may grow 1' or more sometimes, especially if planted early in your growing season. When planted close to the summer time, it is possible that it may take a break on its growth spurt because the temps make it do that.