Huge hydrangea

Hi all.... Hope ur well and your gardens are rewarding all your hard work and effort.... I have a very old hydrangea in my garden which until this year has never really grown much.... See now following Monty's advice I have been leaving the flower heads on and looking after it a bit more. This year it has exploded. It's amazing it's smothered in blue and pink flowers and has grown huge spilling over the lawn by at least 2 feet. i am going to have to give it a severe chop in the spring but how hard ( and I realise it will affect next years flowers)  and how far can I chop it back? Help x