USC's size usually isn't too apparent to me. Class size depends often on the subject- my science classes have always been rather large, but most of my english classes next semester are 15 people. I've never felt particularly intimidated by the size of the classes. If the classes are large, they will generally split them up into discussion sections or labs, which are much smaller groups. I've definitely had plenty of one on one attention from professors- that is mainly a question of your own initiative. All professors have office hours, but you have to make the effort to go to the office hours. When you do, you are certain to get good personal attention.