The relative size of a neonate impacts many aspects of prenatal and postnatal surveillance and care. The designations of appropriate for gestational age, small-for-gestational age, intrauterine growth restriction, and large-for-gestational age are systematic categorizations used to assess and monitor growth throughout pregnancy and delivery. Each abnormal growth descriptor aids in anticipating neonatal needs after birth because each has the potential for complications related to feeding, glucose utilization, short- and long-term growth, and development. Maternal risk factors that impact the neonate's size-related can have immediate implications in the delivery room as well as significant effects postnatally. Caring for neonates at risk for size complications requires knowledge based on prenatal and postnatal complications. Neonates must be carefully measured and plotted on growth charts to confirm a visual assessment of size. Each growth complication requires individual attention to detail and careful planning to maximize adequate postnatal growth and nutrition. Size matters when it comes to the health and welfare of neonates. Anticipatory guidance can improve outcomes in the neonate at risk for failure to thrive from size complications at birth. Part 1 of this article provides an overview of the size classifications and a discussion of clinical factors that are associated with or contribute to small-for-gestational age births. Once the neonate's size for gestational age is calculated, a focused physical assessment is described along with nursing care and prognostic implications. Part 2 will focus on the physical assessment, nursing care, prognosis, and complications associated with large-for-gestational-age neonates.