Several metrics exist for matching the size of donor to recipient in paediatric cardiac transplantation. Different centres employ different metrics for size-matching to determine the viability of donor hearts. Inconsistent evidence exists, with limited consensus as to the metric for size-matching that is most predictive of outcomes after cardiac transplantation. Furthermore, any metric must function within the tight timeline available for the assessment of the suitability of the donor. At the time of the writing of this paper, the most commonly used metric for size-matching in paediatric cardiac transplantation is the donor-to-recipient body weight ratio. In this article, we review published literature evaluating commonly used metrics for size-matching in paediatric cardiac transplantation, including weight, height, body surface area, and imaging parameters.