I take it you have had a scan to be told that? -They don't normally scan until 12 weeks and the first scan is the dating scan....if the scan is showing 5cm I would suggest you are further on then you think. I don't think they can tell at this stage if the baby is going to be big, infact they're not very good at guessing the size at anytime! They told me (at 36 wks) my baby was 9lbs already and was going to be 11 + lbs when he was born and he was 9lb 8.5oz and 11 days over due!
Good luck - and Congratulations
I have a question to ask. Please help! This will be my first baby. I'm 5 weeks, and the doctor told me that the baby is already at 5cm. And I have read articles that the baby should be about 2.5cm. Tell me, Am I going to have a bid baby??