It's a large enough advantage to where the rowing club coach at my school told us that schools heavy into rowing such as Harvard won't even consider you if you're not the minimum required height to row for their school (It's somewhere over 6' something).That does make sense. I'm sure its not too large of an advantage, though.
I never thought that it would matter or perhaps, I just didn't care since I only do rowing for fun and as a form of exercise at the same time. But I have a friend who rows for competition and he's just around 5'7'' and I'd say that he performs as good as the others in his team.