Hi Tom. Without getting specific since I don’t truly know you or your circumstance I can say that generally, by performing unilateral exercises that focus on your lats/upper back for your right side, you can begin to develop more symmetry in both size and strength. Perform more repetitions/sets concentrating on your right side (but of course not neglecting your left side) to help develop your right lat more. Whenever I’m faced with a client with asymmetries such as you describe, I utilized a lot of unilateral exercise to shore-up the weaker/non-dominant side.
I hope that this helps.