Get a new doctor. Seriously.
I'm 4'10" and about 113 pounds pre-pregnancy. I carried and birthed my son vaginally with no drugs just fine. My friends who are like 5'5" and more like 150 pounds prepregnancy had a lot more problems then I did.
Unless you are technically a midget (under 3 feet tall) then size has little to do with it. Your doctor is simply lazy and cesarean happy.
A cesarean is not something you want. It takes way longer to heal, you have way more risk of infection and even dying during a cesarean then vaginal birth, it's more dangerous for the baby to be cut out then born the way nature intended. The healing is a health forum, you can't lift more then like 8 lbs. You're doped up on painkillers. It makes it harder to breastfeed. You have to stay in hospital for longer.
It's just not good. And generally they're used too much. Wait until you go in to labor and see how things go. It's ridiculous to accept one now when you don't even know. For all you know you'll have a vaginal birth in under two hours, one of my friends did and it was her first child.