Accurate transient and disruption cavity dimensions are critical for estimating the energy release associated with impact. Transient and disruption cavity size can, in principle, be inferred from morphometric relationships based on crater diameter. However, locating the crater rim can be difficult for eroded terrestrial craters, and existing morphometric relationships are mostly based on observations of extraterrestrial craters where morphologic features at best provide imprecise constraints on the collapsed disruption cavity margin. Fortunately, magnetic survey data collected over terrestrial impact structures demonstrate that collapsed disruption cavity size can be estimated directly from changes in the magnetic anomaly character. A lower bound on this parameter can be defined by the outer limit of short-wavelength, intense magnetic anomalies produced by impact melt and/or suevite deposits. An upper bound is given by the inner limit of magnetic anomaly trends associated with the pre-impact target rock configuration. Using published values of crater diameters (D) and values of collapsed disruption cavity diameters (DCDC) derived from magnetic data for 19 complex terrestrial impact structures, we derive the relationship DCDC = 0.49D. These data and the possibility of geometrical similarity in crater collapse suggest that this relationship is independent of complex crater size over more than a decade of size variation.