The size stated in plant labels is supposed to be for a typical plant at 10 years of age, not when the plant reaches maturity and "dies of old age". Use that as an estimate, not as something written in stone. You can probably start a huge storm by simply asking people how accurate these labels have been for them.

Since hydrangeas can grow past 10 years of age, their size at maturity can be larger than the one seen in plant labels. So, can Alice get bigger than 6x6'? Yes, my Monrovia Label says 8-10'. Now ask in what zone was this tested? Good question; the label does not say.

Luckily for you, your zone 5 cold weather will help keep their growth more subdued than my more temperate zone 8 climate. What do you do if they exceed the planned size at any time? Prune them.

Luis