There is no regulation on this. 15 feet is not excessive. Based on 6 air changes per hour, a 70 CFM fan would be "adequate". I always prefer to use 100 or 110 CFM even for a small room like your. This will minimize any problems with mold and mildew. Spend the extra $$$ and get a unit with a maximum sone rating of 1.0 You will live with the noise of this fan for many years, so it is worth a few bucks to go quiet.