At 6'3", you do not have to worry about getting "too big." At all.
Even if you were to get up to a lean, muscular 240 or 250 (which, to be honest, would be very difficult at your age anyhow), you'd look like a pro athlete, not some clunky dude who's "too big."
Do you just swim and run, or do you do any other sports? Michael Phelps is 6'4" and around 195. From a physique-perspective, he looks lanky and scrawny as hell. 9 times out of 10, more lean muscle and increased strength makes athletes better.
It's actually called the "Men's Physique Division". The pic above is from a recent contest and the dude on the right won. To get a build like theirs, lift weights 3-5 days a week, do cardio 2-4 days a week, and eat well... just like a "regular" bodybuilder.
At your current size, are you kinda lean or soft/pudgy?