It depends on the type of hydrangea, how big it is to start with and what the estimated size at maturity in the plant label says. Anywhere from 14 to 18 inches may be fine for Big Leaf Hydrangeas in 3 gallons containers. But smaller if it is a cutting and then upgrade once it gets root bound. A Limelight Hydrangea would need eventually a half wine barrel or may need to be planted in the ground in full sun. Consider using dollies if it needs to be brought into the garage/basement during winter/when freezing to ensure spring blooms on old wood.
