For the most part other than balancing size, i.e. weak legs or back you can't train for symmetry. Your flow, lines, muscle attachments are all genetic. You can work with what you have but you can't change you basic muscle structure. High Lats don't become Low Lats and great biceps peaks are not something that come from concentration curls.
So for those who say don't focus on size but instead focus on muscle structure I say "Sorry, it doesn't work that way".
While I do believe that a few of the elite bodybuilders are overly muscular for their frame I don't believe that anyone posting on this thread has to worry much about that problem now or likely ever in their lifetime.
You are much better off focusing on adding as much quality muscle to your frame as possible and letting your genetics decide whether you are going to look good. We can't all be pretty but we can all be strong.