There is every chance that this baby could have been smaller anyway and nothing to do with the smoking hun. It is a very real concern that you have and I think that you should go and get the scan just for peace of mind if nothing else. Be aware though that the size guesstimate can be way off, but I would ask for them to check the placental function as this is what is nurturing your baby.

And not all smokers have small babies - my mum smoked with all of us and her smallest baby was her second bub at 8lb and he was 4wks early - the rest of us ranged from 8lb 13 up to 10lb 14 (there were 5 of us), small consolation I know when you are struggling with giving up. Good luck, I'm sure your baby will be fine.