"I was wondering if anyone knew the max height and diameter of an American Chestnut."

The answer to this would be NO! One would have to rely on old, unsubstantiated reports and would have no way to really know if they were correct.
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Examples of reports thought to be reasonable can be found in resources such as The Native Tree Society's website. For more info about these entries, see the full article, linked below.

North Carolina
American Chestnut Joseph S. Illick stated that a chestnut at Francis Cove, near Waynesville, Haywood County, had a trunk diameter of seventeen feet. This is considered the greatest known diameter of any eastern hardwood.
-ref.: Grimm, William C. 1967. Familiar Trees of America, p. 109.
-ref.: Detwiler, Samuel B. "The American Chestnut Tree," American Forestry, October 1915.
-Comments: Randy Cyr of ENTS reported (1/18/2004) that he had interviewed a relative of the tree's owner who knew the stump as a boy, and said the tree was hollow, single-trunked, and was cut for firewood, but any photographs had been lost.

American Chestnut A. J. Sharp reported an ankle-high stump of a chestnut in the Greenbrier section of the Great Smoky Mountains National Park that was "13 feet the long way across."
-ref.: Grimm, William C. 1967. Familiar Trees of America, p. 109.
-Comments: It seems curious that the tree was cut so low. Perhaps the trunk was hollow, and was cut for firewood, or was on a slope, and was cut low on the uphill side.

American Chestnut Girth 33' 6", height 75'. "Great Smoky Mountains." Lane does not indicate whether this tree was in North Carolina or Tennessee.
-ref.: Lane, Ferdinand C. 1953. The Story of Trees, p.67.
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Tennessee
American Chestnut Circumference 28 1/2 feet at four feet above grade. Located at Porters Flats, Greenbrier Cove, Great Smoky Mountains National Park, Sevier County.
-ref.: 11/19/1933 photo, Flora and Fauna Section, Albert "Dutch" Roth Digital Photograph Collection, University of Tennessee. (via Internet).
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New York
American Chestnut Girth (1921) 26 feet. Located at Esopus (ca. 15 miles NNW of Poughkeepsie). Tree died before 1927.
-ref.: Illick, Joseph S. 1927. Common Trees of new York, p.12.
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Maryland
American Chestnut A huge chestnut cut in 1895 by a tenant on the farm of D. H. Zile was eleven feet in diameter. The trunk and larger limbs were dynamited, and the tree yielded fifteen cords of firewood. It was said to be "the biggest tree in the county, if not the state." Located near Taylorsville, a community on South River, Anne Arundel County.
-ref.: "One Hundred Years Ago." The Baltimore Sun, 5/21/1995, quoting from an article in the American Sentinel of 5/18/1895. (per Internet)

Here is a link that might be useful: Great Eastern Trees, Past and Present by Colby B. Rucker

Current living official record holder per the American Forests' 2011 National Register of Big Trees:

Nation
American Chestnut - height: 70', circumference: 288", crown spread: 70', total points: 376, location: Clarkston, WA

This tree is in Nova Scotia, canada. It's fairly large, and as it's only about a hundred years old, it will be bigger yet.

[IMG]http://i459.photobucket.com/albums/qq314/JocelynClarke/Ashdalewithcarforsize.jpg[/IMG]

Hybrids with Chinese chestnuts are often more limby and shorter in the F1 generation. It takes about 3 generations of backcrossing to get American character back again. There are a few timber type Chinese selections, which hurries the process along, but finding a timber type Chinese is difficult. There are two nice ones in Wolfville, Nova Scotia, and the crosses look good in the F1's. The F1's have the red twigs of the Americans, and longer leaves than the Chinese...and intermediate blight resistance.

Here's the Nova Scotia tree, link reformatted:
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https://umaine.edu/news/blog/2015/11/25/tallest-american-chestnut-tree-in-north-america-discovered-in-lovell-maine/