Emerald Green Arborvitae.. how big can I go?

Don't these trees get up to 20 ft or so?

A 22 x 17 container is roughly equivalent to a 25 to 30 gallon trade nursery plant container and those routinely are used for larger 6 feet + trees for planting in the landscape. If the rootball of the 5-6 ft arborvitae will fit in your container then you can certainly plant that size. The arborvitae will ultimately grow to a size that the root mass will support when it fills the container.

What you need to know it that to keep the trees in the best of health every 3 years or so, once the root mass fills the container, you’ll need to remove the tree from the container to prune the roots (just research root pruning container trees/shrubs), remove about 25% of root ball, sides and bottom, and return it to the container filling the void with new potting soil.