Characteristic lengths govern processes related to flow in unsaturated porous media, e.g., during drainage or evaporation. One can estimate these lengths, i.e., air-entry pressure head and critical capillary head, from either a water retention curve (WRC) or a hydraulic conductivity function (HCF). We provide new analytical expressions for the characteristic lengths based on a linearized analysis of the WRC and HCF curves. These are then used to investigate the impact of model selection on length-scale estimates, to assess their sensitivity to model parameters, and to provide guidelines for the choice of the alternative expressions under flow scenarios of interest.