The length is regulated by the National Firearms Act of 1934... 16" or longer barrel (18" for shotguns), 26" or longer overall length with foldable stocks extended, or else you get to pay thousands of dollars and spend ten years in a federal prison, and lose the right to ever own a firearm again. Period. The AWB doesn't affect the NFA. The measurements, however, are taken with stocks fully extended, so if you had a firearm that had a 5" receiver and a 16" barrel beyond that, but a 12" folding stock, it would be 33", not 21".
In any case, I repeat in case anybody thinks the AWB changes the length requirements: It does not. You have a rifle or shotgun that does not meet the length requirements, and you do not have it registered to you as an NFA firearm, you WILL be sent to jail for ten years on a felony. End of story.