Hi;
The girls tend to stop growing in height by about 7- 8 months, or when they have had their first heat. The boys often mature more slowly and often don't stop until close to 18 months.
Jrt breeders have a method which is fairly accurate for testing what the mature adult height will be. When the pup is 4 - 4 1/2 months old, we measure them from the floor to the top of the shoulders. We then add 1 1/2 inches to that height, that should be a reasonable guess as to how tall the dog will be.
I suggest that the pup be measured several times during that two week period. Measure them on a flat surface and make sure the legs are balanced under the pup (not stretched out behind etc). Make it a nice experience and don't manhand the pup too much, that will only make them either stand with a roached back, or slink. I often have them stand in front of my fridge, use a ruler and a pencil to mark (which can then be easily wiped away).
It is a reasonable guide line, should tell you within 1/2 inch the size.
Good luck
Lynn