I've hiked there a bit myself!
https://www.flickr.com/photos/miguelvieira/6142136271/
Most of my data came from Eastern Old-Growth Forests by Mary Bird Davis. She defined old growth as forests that were never logged or that are so old that there is no longer any evidence of logging.
The thing with many eastern forests is that because of either their species composition or their site conditions (soil, climate, etc), their trees will never get very large.
What I remember about the Smokies is that no one knew exactly how much they were logged, but the National Park Service estimated about 60 to 70 percent. Most of the remaining unlogged forests won't really have huge trees. In fact, some huge tulip trees that are popular hiking destinations are less than two hundred years old, while small hemlocks on ridges can be over three hundred years old.