If you look at the AKC standard, beagles are described as either 13" and under or not exceeding 15".
I have had 13" beagles from both hunting stock and from conformation stock. Missy and Patty were both from hunting stock. Dulce and Macy were from conformation -- specifically from Mildred Austin's breeding.
I have always been fascinated by the thought of pocket beagles. According to beagle lore, they were 8" beagles. I read about them the first time when I first got into beagles, and every knowledgable source agrees that they did exist and that their bloodlines are extend. There were three registered kennels and they were all wiped out by a communicable disease, I believe distemper.
Here's what I have found on the internet:
http://www.beaglepro.com/pocket-beagles - who compare well bred 13" / 15" with advertised pocket beagles.
https://www.foxfirebeagles.com/pocketbeagles - has much of the same content as the above source.
https://www.barkleyandpaws.com/dogs/breeds/do-not-buy-pocket-beagles Explains why you should not buy one.
From the Beagle Club of Western Canada: https://beagleclubofwesterncanada.weebly.com/pocket-beagles.html
If you do further research, many "breeders" are calling the 13" beagles, pocket beagles for marketing purposes. However, a 13" beagle is not as small as the original Pocket Beagle kennel dogs.