Like others have said, every baby is different. But from anecdotal evidence, if both parents were big babies and are tall people, it seems the babies will be big too (assuming they are full term). Likewise, if both parents are short the babies tend to be smaller. A BIG generalisation I know, but it seems to work.

An example: I am 5 foot 8 (so not short) and DH is 6 foot 6 (MASSIVE) so our boys were both on the large-side and neither were in 000 for more than 2 weeks. I was given 0000 clothes as gifts but these went straight to the Salvos. (My eldest who is 4 in June is now in size 6 t-shirts LOL!)