I would give your concerns to your doc more so. Some women tear others don't. Some tear with one pregnancy others don't. If you were to have to have the baby vaginally and your doc saw that you are to small he'll prolly cut you a lil to open you wider.
My friend told it's about baby's head circumference abd abdominal circumference and your pelvis that drs worry about the most when it comes to size but I really don't know because I can't deliver vaginally for other reasons. Have you see your baby's estimated measurements? Maybe you could talk to your dr about them.
You'd be surprised what you can squeeze out of that thing. Most people tear because they are pushing too hard and too fast for their body to stretch correctly. If your used perinatal massage and you push slowly, you more than likely won't tear, no matter the size of baby.
Im a little person myself and I had a vaginal birth with both of my kids and will this one too. I tore with my first and had an epidural with him I had my second all natural and didnt tear at all. So some people tear some dont. My sister is bigger then me and tore with two of her kids so I wouldn't put the size you are it really doesnt matter! :) hope I helped.
If you too small the doctor will cut the bottom of the vagina open to give you more space. Its called episiotomy. Then stitch it up after birth