Most difference in testicle size is normal if there is nothing else wrong with the penis and testicles. One can usually be up to 25% larger then the other while young men are undergoing puberty and its not a big deal. It is unlikely to affect your sperm assuming there is no other problem. I’m guessing from your age that you are close to the end of most of your pubertal development so I would probably make sure you point it out to your doctor the next time you see them and make sure they do a good exam to look for a varicocele or other changes. There is no rush and if it waits until your next physical exam that is fine. If your doctor does think there is a problem, they may send you to get an ultrasound done (which is totally painless) to look at the size of the testicles or perhaps to a urologist who is a doctor who specializes in that area of the body.