"Are you a captain?"
It becomes plural if you want to talk respectfully and it also implies some formality. You talk with plurals, for example, when you're talking to your professor, teacher, police officers, a judge or totally strangers. You don't have to, in most cases. I've never seen someone talking to their professor or teacher in second person by using "sen", though. Most people talk to strangers like that, however, unless the other one commands respect by the first apperance, like if she/he looks very rich or powerful, then it becomes "siz". :) Just go with "siz", when you feel the need to show kindness, respect and/or formality.