OB’s are renowned for putting their mouth into action without first engaging their brain. Yes if you and your partner are tall then there is a tendency to have larger babies, but this is only a tendency and not a rule. If you and your brothers were not big and your first baby was not big why would anyone expect this baby to be big?
If you were being measured to estimate your gestation and found that you are on or just behind your dates (which is normal) why would your OB decide to guess rather than measure?
You are sort of right when you say that subsequent babies don’t always engage until labour. Some engage early and yes some do not engage until the start of labour but they don’t usually just float around in there. Their head will be down in the pelvis but just not engaged. By 2 fingers he probably means that your baby’s head is in the pelvic brim and there only 2 finger width sticking out above the pelvic brim. If this is the case then your baby is well engaged.
From what you have said in your post there is nothing to worry about, everything sounds perfectly normal to me.