With that said, it is very common for one testi (or testicle) to be larger than the other. If, as you said, this has been the case since birth, and you have never had any complications arise, then it is very likely that you have no cause for concern.
If, however, you were to experience a sudden change in the size or shape of one  (or both) of your testicles, or if you were to begin to experience any pain/tenderness of the testicles, then I would suggest having an examination. Such a change could be the result of a sexually transmitted infection, a cancerous growth, or a number of other health problems. To be clear, from what you’ve described, I don’t believe this to be the case for you.
You also asked about whether this difference in the size of your testicles could have an impact on your ability to impregnate someone, and the simple answer is no. While the testis are responsible for sperm production, a difference in the size of your testicles should not impact your fertility.
I will note, however, that there are many reasons that a couple could have trouble conceiving a child. If you are concerned about this, or if you have had trouble conceiving, you could make an appointment to learn more about your fertility options.
Thank you for reaching out, Lukas, and if you have any further questions don’t hesitate to contact us again.
Matthew