To get bigger flowers, cut all the way back. In late winter or early spring you can cut all the way to the ground.
Here is a great article on it: https://www.finegardening.com/article/pruning-hydrangeas
Don’t cut them ALL the way back. You won’t get flowers for @ least a year if not more. The flowers grow off the old growth, so if that’s what you cut it off it won’t bloom. Cut only small amounts off. Look online for detailed directions. Cut only dead growth back to the new green buds coming out.