We develop a self‐consistent scaling model relating magnitude M_w to surface rupture length (⁠L_E⁠), surface displacement D_E⁠, and rupture width W_E⁠, for strike‐slip faults. Knowledge of the long‐term fault‐slip rate S_F improves magnitude estimates. Data are collected for 55 ground‐rupturing strike‐slip earthquakes that have geological estimates of L_E⁠, D_E⁠, and S_F⁠, and geophysical estimates of W_E⁠. We begin with the model of Anderson et al. (2017), which uses a closed form equation for the seismic moment of a surface‐rupturing strike‐slip fault of arbitrary aspect ratio and given stress drop, Δτ_C⁠. Using W_E estimates does not improve M_w estimates. However, measurements of D_E plus the relationship between Δτ_C and surface slip provide an alternate approach to study W_E⁠. A grid of plausible stress drop and width pairs were used to predict displacement and earthquake magnitude. A likelihood function was computed from within the uncertainty ranges of the corresponding observed M_w and D_E values. After maximizing likelihoods over earthquakes in length bins, we found the most likely values of W_E for constant stress drop; these depend on the rupture length. The best‐fitting model has the surprising form W_E∝logL_E—a gentle increase in width with rupture length. Residuals from this model are convincingly correlated to the fault‐slip rate and also show a weak correlation with the crustal thickness. The resulting model thus supports a constant stress drop for ruptures of all lengths, consistent with teleseismic observation. The approach can be extended to test other observable factors that might improve the predictability of magnitude from a mapped fault for seismic hazard analyses.