Hi there -
I think what you are describing sounds totally normal, and I applaud you for paying attention to your body that way. It is totally normal for testicles to be different sizes, just as most women have one breast that is larger than the other. Many people have a foot that is larger than the other, you get my point.
That said, the average age range for testicular cancer is 17-32. You are very close to that. If you have always had a testicle that is larger than the other, and there is no pain or no lumps, I wouldn't worry much about it. But keep doing exams. I am sure there are many sites that would give you a great description, and possibly pictures, on how to do a thorough testicular exam.
Again, congrats on being so self-aware. This will serve you well in the long run. Good luck.