Pistol Length Question

[Shiz]

I have been searching a while, and not finding much success.
A leading NFA dealer near me told me that an AR pistol had to be under 26.5 inches. If it was over 26.5" regardless of barrel length, it became an AOW and a $5 NFA tax/wait had to be paid.


Does an AR pistol have to be less than 26.5 inches?
I was under the impression that an AR pistol could be any length (barrel any length too) as long as it didn't have a stock, or a VFG. (maintained designation as a one hand fired firearm)
I have been searching the ATF website and cannot find the exact passage that explains this. I don't care about being wrong. I just don't want to be misinformed, as I was thinking about doing an a5 pistol tube on a 12.5 or 14.5" barrel.

Any help would be appreciated. Direct law citing would be great too.
Thank you.