Functionality of the testicles is overwhelmingly more important than size. The testicles have two functions: they produce spermatozoids and produce the hormone "testosterone". A Decrease in the size of a testicle can lead to a loss or decrease of functionality. This is called "testicular atrophy". You can visit your doctor and request a sperm analyses (spermogram) for an evaluation of the number of spermatozoids present in the semen, as well as observe their functionality. You can also check the blood level of testosterone. If the number of spermatozoids has decreased and there is a low level of testosterone in the blood, it is likely that you are experiencing testicular atrophy. Testicular atrophy has many possible causes and in the case of such a diagnosis, you should consult a urologist or ean ndocrinologist. If everything is normal, you have no further need of measuring the size of your testicles.