I too worry about our little one's size. I know I was tiny but my partner was 10lb something. Are measurements just judged from when they measure your fundal height? I had my first measurement last time. She said they use the rule of thumb that it's a cm for each week. I was measuring 27 at 25. But then I expected it to be bigger because my BMI was a little high when I got my BFP (we weren't really expecting things to happen so fast). Not sure if that's the case but it did worry me a bit.
Are you having a 4D scan?
Depending on the sonographers experience they may guesstimate the weight for you based on growth
Mine is a qualified MW too with 25 years experience and has predicted us at around 7lb 12oz based on her average growth at 28 weeks (she weighed 2lb 13oz roughly at that scan) I've heard from reliable source that's she is rarely wrong
Some might give you an idea if you ask nice
PS updating list when I'm on PC tonight... iPad isn't as easy