The height given for rhododendrons by sellers is the height that could be reasonably expected after 10 years of growth. Given enough time nearly all rhododendrons except those designated as dwarf will exceed 6 feet.

You can search for the taller ones on the American Rhododendron Society plant finder pages. Using it you will find the tallest hardy ones after 10 years are:

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Caractacus, red, -25F

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Lady Eleanor Cathcart, pink, -15F

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Spicy Nutmeg, pink, -20F

Here is a link that might be useful: ARS Multi-Parameter Search for Rhododendrons

Rhododendron maximum is one of the largest species. Its native to the Appalachian mtns in the southeast and mid-atlantic states. The flowers are usually white.

Cynthia gets big and blooms here at the end of May in to June. This one is 35 years old.

Here is a link that might be useful: Rhododendron, 'Cynthia'.