If they are simply small but still descended, there should be no effect. One seminal ejaculation contains enough sperm to impregnate a million women or something like that, nature is very prodigious. So even if someone's testicles are a trifle on the small size, they will still produce plenty of sperm. If you are saying the testicles are undescended, that is a different question. They need to be out of the body cavity to be cool enough not to kill the sperm with body heat.
I have heard that testicles played major role in reproductive system and produces sperms, if they too small rather invisible then what will be the position of fertilization and what other factors are involved in production of sperms
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