My daughter is five months old now and her head circumference is 42cm, does it imply she has a big head?

No, it's 50th-75th : %ile, depending on use of the who or cdc growth charts. More important than size ( within the normal range) is velocity of head growth, which reflects brain growth; i.e., is it ⬆ing rapidly, dropping off or following a growth curve? The aap recommends developmental surveillance with standardized screens at each check-up. See the aap healthy child app & firstsigns.Org for details.