Hi.
Thanks for your query and an elucidate history.
Your history of a kick to the scrotum 10 years ago is self-explanatory.
BY a direct trauma over the testicle, the blood vessels may get blocked , particularly the arterial supply to the testis by pressure of
hematoma or spams due to trauma.
Over the time, the
fibrosis sets in. Fibrosis shrinks everything . This happens in testis so easily as it is very fragile, blood supply is tricky and comes from a long place near the kidneys, it is very sensitive to temperature variation, usually the temperature of the testis is 2 degree celcius below the normal body temperature.
Since you can have a normal sexual function, at least one most important part of life is fulfilled.
I would advise you the following:
Semen analysis.
Ultrasonography and color doppler of the scrotum.
blood
testosterone levels.
Remember there is no way the size of the testis can be improved.
But if all the investigations are normal except the size of the testis, carry on the normal life and stop worrying about it.