Well, let's remember that there is both muscle size, and muscle strength. So size isn't the only variable, unless you're talking about what's discernable in a picture.
But yeah, it is possible to isolate the long or short head, vary the width of your grip on a barbell and you'll see a difference (over time). I doubt the overall aesthetic would be vastly improved by having huge short / small long though or anything like that.
Also, when I think of well rounded bicep development I think about how they fit into my overall physique, naturally I compare them to my triceps, so that there's no undue stress on my arm or elbow. Having huge bi's is a common goal of many a gym-goer, but that doesn't mean it should be your only goal.
Anyway, that's my opinion.
For the purposes of this discussion I am only talking about appearance. I certainly agree that strength is important and that it is important to have balance.
What I am trying to understand is the logic behind 'hitting a muscle from different angles' and trying to emphasize different parts of a muscle. Can a muscle selectively contract fibers in a certain region of a muscle?
I see a lot of people trying to shape their muscles by using different exercises. I thought all this was nonsense, but I also try to regularly reevaluate what I think I know.