Will I have to have to have a c-section if I am having a really big baby?

Possibly. Talk to your obstetrician. The main concern is that the head and body will not fit through the pelvis and be able to be delivered naturally. A frank discussion with your obstetrician before labor will help you sort out what is best for you and the child.
Yes. Big is a relative term. Even though most really big babies are delivered via c-section, sometimes big babies also deliver vaginally. Chances of vaginal delivery are good if you have gone into labor spontaneously, labor has progressed well, if you previously delivered a big baby without problems, if you have no diabetes, if you have not gone beyond your due date, and if you have an average bmi.