The are many definitions of appropriate sizes out there; do a search in the forum for 'pot size' or 'size' to see many postings on the subject. The pots I have are about 18-24" and they do well in those. If your plans are delayed and you do not plant the shrubs rightaway, I would put them in bright shade because the plasticky pots get hot with direct sun once temps in your area get to the mid-high 80s...

Suggestion for long term growing of hydrangea in containers from plantsman Dan Hinkley - he rarely if ever gives other than spot on advice...

"Move a gallon sized plant directly into a 15 - 25 gal container using a bark based soilless medium available at garden centers. Water and fertilize as for those plants in the ground and replace the growing medium every three years or as needed."

Good luck if you do no better at converting inches to gallons held as I can ;) A shorter or tapered 24" diameter container may hold 15 gallons. If it's straight sided and deeper it could well be the 25....

3.14*r*r*depth = volume in cubic inches where r is the radius (half of the diameter of the pot).

divided by 231 (c.i. per gallon) = number of gallons

But the pot is semi-conic where the bottom is typically smaller than the top. In this case measure the bottom add it to the top and divide by 2 to get the average diameter.

So a 24 inch pot with say an 18 inch bottom would have an average diameter of 21 inches.

If the pot was 24 inches deep then the volume in c.i. would be 8312. Divide this by 321 and get nearly 26 gallons.

Since you will be building the containers yourself, I'd shoot for a 24" box. This will accommodate even a large hydrangea for a good number of years without the need to root prune frequently or refresh the soil. Be sure to use a very high quality, durable potting soil and add in a slow release fertilizer when potting. Plants grown in containers will need more frequent fertilizing than those in the ground.