How big must a baby be for them to do a c-section?

I'm 32 weeks pregnant with my second and I just went in for an ultrasound and the baby was 4.3 lbs (1.944 kgs), which is bigger than average. Now that might not be alarming for most people but I'm a really tiny woman. I was 95 lbs and 5.1 ft before pregnancy. My first baby was about 5.3 lbs when born. I'm so scared this baby will be huge. I really don't want to have a c-section, but I'm just wondering how big the baby needs to be for them to decide you should have one? I know babies typically don't get too big for you to push out, but still. Since I'm smaller than the average woman you can't follow the guidelines for what works for an average woman for me?!