what are the largest stars in the HR diagram called?

Answer 1

http://www.google.com/search?q=stars+in+the+HR+diagram&hl=en&prmd=imvns&tbm=isch&tbo=u&source=univ&sa=X&ei=eKxfT8jvOOSd2QXboZmzCA&ved=0CCUQsAQ&biw=711&bih=453