The above chart shows you the categories of "mini" cows at 3 years old. The best "estimate" of how large your cow will get is based on the size of the mom (Dam) and dad (sire). There’s no magic measuring tool to ensure your cow is going to be any specific height. 

We can ESTIMATE, but just like humans, cattle come in various shapes and sizes. Factors like genetics and nutrition play essential roles, making it important to remember that while we can predict, each miniature cow is unique.

Never trust a breeder who will guarantee you a cow won't get over a certain size.  We do our best to "predict" based on years of experience and past calves.