I dont think your height has anything to do with the capacity of your pelvic muscles or the power of labour hormones!!
I would be asking for more proof than just your height if you are worried. Thats a bit of a copout from your OB in my eyes, but I'm sure s/he can provide you with all the information you need to make an informed decision.
If not, I'm sure there are plenty of resources available that can help you do make your case.