An old specimen I have watched grow for many years in zone #5 is roughly 4 by 4 feet. Can reach much larger in the warmer end of it's hardiness zone.
Maintenance
I like to dig a large hole, not to deep and mix the soil with 50% peat moss, compost or other organic material. Plant in a slight mound for good drainage. Add a cup or two of Ammonia sulfate (or any other acid producing fertilizer) and some regular fertilizer. Water in good and keep moist. Mulch will help to slow down the loss of water. I like to add acid fertilizer in the fall and spring. Also you can use a liquid acid fertilizer such as Mir-acid. Any pruning should be done right after flowering.
Cultivar Origin
A catawbiense hybrid Rhododendron by A. Waterer.
Notes & Reference
#144-Missouri Botanical Gardens web site (www.missouribotanicalgarden.org), American Rhododendron Society