I just had my third boy. My first was a week late and 9 lbs. I was induced and had only dilated to a 3 when his heart rate dropped and they couldn't get it back up. They did an emergency c-section.
He had a big, very round head, and hadn't even begun to drop.
So with my second, I wanted to try a VBAC. But at 35 weeks, I was HUGE! I'd outgrown all the clothes I'd worn at the end of my first pregnancy, and I still had 5 weeks to go! So they did another ultrasound, and estimated that he'd be 10 lbs by his due date. They scheduled me a week early, and he was 8 lbs 12 oz. He could have been almost 10 lbs a week later, but he really wasn't as big as we'd been expecting. The one measurement I thought would be more reliable was the head circumference, but the u/s said that it was measuring very big, 41 weeks big at only 35 weeks. But his head really wasn't very big at all, especially compared with my first. But the u/s was a great predictor of his general size, because he's a big, big kid! He's above 97th percentile for height, while my first is ~75%.
With my third, who is 4 months old now, I had a totally different pregnancy, so I was sure it was a girl. NOPE! I didn't feel any different about my size until near the end. I just didn't feel as big, and I definitely wasn't as HUGE as I was with my second. Still, we were quite surprised when he only weight 7 lbs 7 oz! He's tall, though, at 90th percentile for height.
Okay, so those might all sound big to you, since you're hoping for something in the 6's. But yes, I could tell the relative size of my babies, but only in the last 2-3 weeks. You should expect to look a little bigger with your second, even if they're the same size, because you're all stretched out already.
And you can also guess a little bit by the percentiles of your son. You're not likely to get one child well below 50% and one child well above 50% (although it could happen).
Good luck, this is always a difficult decision to make. I really wish I could have had a VBAC!