Beagles  that are under 13" and those over  13" are sometimes in the same litter. They are not a separate breed.  The size designations are for AKC showing purposes only.  In Canada . England , Australia and most other countries the beagles are not divided by size and all compete in the same classes.  The AKC limits the showing size to 15" but the English standard  does not have a disqualification and has a preference that they beagle be between 13" and 16".
Some beagles do actually grow beyond 16".

After all my years of trying to predict the ultimate size that my puppies will grow to be I still  can make only an educated guess. 

I have made a habit of taking pictures of my 8 week old puppies while they are stacked on a board that has alternating black and white 1" marks visible. I can measure size and proportion by using a caliper on the puppy and then comparing it to the marks. As a general rule I have found that ones that are less than 7 1/2"  may stay under 13" and those over
8 1/2" usually end up over 15". 

8 1/4" grew to 14 7/8"                   7 1/2" grew to 13"                      8 1/2" grew to 15"

All three of these 8 week old puppies were from the same parents. The first two were in the same litter.  They each completed their AKC championships