Hello and welcome to HCM.
As an
Urologist, let me opine that, a significant change in size of testis, suggests that in the past, very likely, you may have had an inflammatory change in the testis.
If the other testis is normal, then you need not worry.
Sperms will be produced in normal range from that testis, so
fertility is not an issue. As male hormones,are also usually normal, erections and hence, sexual functions will be normal.
You can get it confirmed, if you're anxious, by doing a semen test.
If you've any other doubt, you can send the question here, in my name.