The recording on high‐resolution broadband seismic networks of several great interface subduction earthquakes during the last decade provide an excellent opportunity to extend source‐scaling relations to very large magnitudes and to place constraints on the potential range of source parameters for these events. At present, there is a wide range of uncertainty in the median rupture areas predicted for any given seismic moment by current relationships between magnitude and rupture area for subduction interface earthquakes. Our goal is to develop an updated set of earthquake source‐scaling relations that will reduce this current large degree of epistemic uncertainty and improve the accuracy of seismic‐hazard analysis and the prediction of the strong‐motion characteristics and tsunamis of future subduction earthquakes. To achieve this goal, we compiled a database including slip models of interface earthquakes that occurred worldwide with moment magnitudes ranging from M 6.75–9.1. We characterized the seismic sources based on well‐established criteria to estimate the asperity areas as well as the average slip on the faults, and we used these parameters to compute an updated set of magnitude scaling relations of the various characteristics of the fault. Additionally, we followed an alternative approach to quantifying slip models for use in developing characteristic slip models of future earthquakes. This involved analyzing the 2D Fourier transforms of the slip functions in the compiled database and deriving a wavenumber spectral model of the slip distribution.