Hi there!
Slight discrepancy in the size of the two testicles is normal and should not be anything to worry about. With infections/ inflammations ruled out possible causes of the pain that may need to be considered include trauma/ injuries, neuro-muscular causes, referred pain from other regions etc. I would suggest getting this evaluated by an internist initially and depending on the cause diagnosed/ suspected, it can be managed accordingly or specialist care may be sought.
Hope this is helpful.
Take care!
The heading should be read as "19 Years chronic pain in testicle"