R.R.
I personally will not attempt a VBAC because I have CPD with other factors, I'm pregnant and going to have a repeat cesarean. But one of my friends had her first baby posterior and she did end up getting him out after pushing for about 3 hours. She just had her second baby and he was actually bigger and she pushed him out easily and he was much bigger than the first baby. She felt that during her pregnancy, her hips/pelvis moved much more this time than with her first. She said she had a lot of pain in her hips toward the end and never experienced this with her first. So even if your baby is bigger, it may be that your body "could" function better this time. I've done a lot of reading about VBAC and the conclusion I came to is that you should ask yourself a few questions. How will you feel if you don't attempt a VBAC (will you always wish you had?). If you do attempt it and it fails, will you still be glad you tried, or will you feel that it wasn't worth putting yourself through it?
What I'm trying to say, is, even if your baby is larger, that doesn't mean you can't do it this time. But you won't know the result until you've been through the VBAC. Try to imagine how you will feel with all of the potential outcomes. i.e. : What if you have a repeat and the baby isn't as big as they said?
My cousin had her baby 2 weeks ago and he was predicted to be 12 pounds by an ultrasound she had only a few hours before he was born. He was only 10-3.
Whatever you decide to do, good luck! I hope it goes the way you are planning. Best wishes!