I think you definitely have the right adea, indianaman_04. A compliment should be accepted as such and thanks given, regardless of what part of the body is appreciated. I think, usually, eye contact will be made and in more times than not words exchanged, but even if these connections haven't been made, a thank you is necessary.

I've had a couple of strangers at the gym ask me how thick my dick is, I think because I'm a short guy and at times it's noticeable. It doesn't offend me and I consider their curiosity a compliment. If there happens to be an ulterior motive behind the question, I think this can be handled satisfactory without any problems.