Calibers on original American Long Rifles really depended upon the time period that they were made...In the decades before the American Revolution it could be any where between .45-.62 caliber...The barrels were made in Germany and bored and rifled over here, as they were made by German immigrants, they were used to making Jeager rifles, used for wild boars...Thus the larger bores...
In the years after the American Revolution, the calibers started declining, by 1820 a .45 caliber was a large caliber and most were being made in the .32-40 caliber range...
Flintlock is right on the money here with his caliber range.
Maybe even shy a couple at each end.
I have seen originals with rifled barrels at least 69 caliber, but that may have been rebored after the rifling was worn.
I have also seen, one, 25 caliber percussion Bedford county rifle. Must have been a pain to shoot.
And that doesn't include the smoothbore rifles that were built in all calibers.
Two books to consult would be the Kindig book and the Johnson book.
I will post the titles when I get home and look at them.