I am not sure if this will help but a hydrangea in a 1g pot could use a container 15" or slightly more. Some of these k hydrangeas tend to spread so buy the pot accordingly. You could try up to 25" for now if you want. Since flower buds are produced only in the summer, be ready to do your thing soon. Flower buds develop here in July. If you are north of me, they may develop as late as August.

Thanks for your reply, luis. Should I move them before or after they set buds? If I recall, last year they didn't get buds until September, and then of course our awful winter killed them all. Once they're dormant and potted up, what kind of care do they need? Weekly waterings?

As for what size pot and care, I usually share what one of my favorite plantsmen (internationally known) says about hydrangea containerized long term -

'For those gardening in less benign climates and wishing to grow hydrangeas, they do respond well to pot culture. A gallon sized plant should be moved directly into a 15 - 25 gallon container using a bark-based soilless compost (meaning potting soil, potting medium). Move the containers into an unheated porch, garage or greenhouse during the winter season, protecting only during the coldest temperatures. Fertilize and water as for those plants grown in the ground and replace the compost (soilless planting mix) every three years or as needed.'

Weekly watering once dormant sounds too frequent to me, but I rarely have plants other than my geranium that must come inside the garage more than a few brief days, the span of specific storms - we just don't have the continued cold here.

Don't worry about the buds too much. If you were to pot in September you could still leave them outside until first frost before bringing them in.

My dormant hydrangeas (in 5-gallon pots) are brought inside the garage in November and not put back out until April. This happens when temps regularly start dropping below freezing at night. Even if the leaves are still on and green, you don't want roots to freeze. Last fall, most of the leaves fell off in the garage.

Mine need watering about four times during that period, so that would translate to every 1.5 months. The roots will absorb and use the water quite well as the pots will become very light by next watering.