Renstar, my mother's babies were ALL "big"--9-10 lb. Except for me at 7 lb 7 oz. She said that her best birth was with her biggest baby, and that it has much more to do with how well supported you feel in labour.
Long painful labours have a lot to do with the baby's position in the pelvis.
Yes you certainly can refuse the sizing scans!! My sister has just had one and is terrified because her due date has now been moved up by TWO weeks. She and her dh are both tall and they think the baby just has a long femur. She is sure of her own dates and I've told her to go by that rather than the scan.