Thank you for your question.
Testicular size is determined by measuring testicular volume. Testicular volume is a predictor of testicular function and therefore of reproductive capacity. Most fertility clinic providers use ultrasound imaging in addition to clinical examination to determine this. Sometimes one testes can sustain injury etc in childhood and appear small. Very often the other normal testes is sufficient to produce the required number of sperm.
If you haven't already, it is advisable to see an andrologist / fertility specialist to determine whether the size and function of testes is normal. They may conduct further tests to help reassure you.