I'm planning on planting 2 of your
|This is a good question. To some extent the mature size will be limited by the size of the container and by the growing season where you are. I would not expect them to reach their full potential size growing in a northern climate such as yours. However, the bigger the container certainly the better, yes. A half barrel size container would not be too big.
I know these are recommended for containers, and they do very well in containers in mild climates. But I should mention I am concerned about your plan to grow them as container plants in Manhattan. In the winter, this will expose their root systems to extreme cold, as well as to oscillating soil temperatures, and the limited soil volume will also make them prone to drying out in winter whenever the soil is not frozen. This is very stressful on plants and in many cases they do not survive the winter outside in a container in cold areas -- such as yours.
This hydrangea is considered winter hardy in your zone when planted in the ground, but I would not count on it surviving in a container outdoors. Also, it is important to understand that it blooms on old wood at the beginning of its bloom season and subsequently on new growth as the summer progresses. This allows it to bloom for a prolonged period including into the fall, hence the name Endless Summer. If it suffers winter damage, causing the old stems to die off, then it can still bloom later in the season on the new growth of the year. So any winter damage will reduce but not eliminate flowering. This might be important if it is placed in a prominent location in your courtyard.
If possible it would be better to bring the containers into a cold but sheltered spot for the winter. Allow the plants to go dormant in the fall, bring them in for the coldest part of winter, then take them back outside to awaken naturally with the season. While indoors, they should be kept very cool in the 40 to 45 degree range so they stay dormant, and the soil should be kept just barely damp -- do not allow it to go bone dry.
I hope this helps.