Your Questions:

*When should I make the move to the containers? The best time is when the plants are dormant and that is usually the winter months.
*How much should I root prune? Is 1/3 ok? It depends on how long the roots are because you want to try and get as much as what will fit into the container. The smaller roots near the top help absorb water.
*How large should the containers be? (please respond in inches not gallons because I really have no idea what that means) 18-20 inches. Gallon amounts refer to the volume.
*Should I put them on the east side immediately or keep them in shade for a while? If they are dormant, it will not matter which of the two places but eventually you may have to provide afternoon shade. Otherwise, give them 1-2 hours of sun or give them indirect sun (like put them at the edge of the shadows). Winter sunlight is "weaker" than summer sun exposure. I have put newly purchased camellias in full sun during winter (until I found/prepared a spot) with no ill effects. Since you are in a different zone, consider this: if you notice that the leaves in direct contact with the sun are turning all yellow, including the leaf veins, then they are getting too much sun.

There are many eons of potting mix solutions that you could use. Many used a pine bark based potting mix. Most suggest that you do not mix in soil from your own garden. Put a mesh and some rocks (on top of the mesh) at the bottom. The mesh prevents critters from getting into the pot from below. The rocks help with drainage; about an inch or so is enough. In places that are hot or windy like your zone, I would also mix polymer gel water beads into the potting mix.

Watering, feeding and sunlight are the issues one usually encounter with potted hydrangeas so once you find a medium that fits, stick with it.

Some nurseries may have around some gizmos with wheels to help you move these heavy pots around as sunlight exposure changes thru the year. Consider buying one per pot and a few extra ones in case they break.

I would also get a soil test done with the garden soil in order to see how to improve it. Local nurseries offer this for free during winter (I try to buy the ammendments that the results recommend from them to make it up).

Hope that helps,
Luis

I'm having trouble finding mesh...any suggestions on where to look?

About the leaves: in the current location, the edges turn brown and crispy, and the veins are yellow. I'm assuming this means they are in too much sun? Please do confirm if so.

And thank you so much for your responses!

If you see the leaves turning brown from the edges inwards, that may signal a moisture problem. Yellow veins are rare and could signal a mineral defficiency (nitrogen for example). If the leaves in direct in contact with the sun turn all yellow or white-ish, that would signal too much sun. An iron defficiency looks differently: the leaves turn light green or yellow but the leaf veins remain dark green not yellow.

Not sure if this is what Louis actually meant or would use, but the type of "mesh" that I have used over drainage holes at the bottom of a container is (aluminum) window screen.

It can be purchased in small rolls more economically than the patch kits.

hth,
Deb

Here is a link that might be useful: example of aluminum window screen