It's a good question, but I can't answer it since I'm not a guy. I guess if I were a guy and it was a choice, I would go with medium sized testicles and be medium sized, but that's not a choice, so I guess I would go with the small testicle choice if I were a guy because I think huge ones would be uncomfortable when they are jogging or sitting down. I'm not voting though.
I heard on House that the size of a primate's testicles was correlated with the fidelity of their females, so primates with very faithful females had smaller testicles and ones with larger testicles had very unfaithful females.