A damage law for dynamic failure in brittle solids is proposed in the present note. The new evolution equation is obtained by incorporating the Grady-Glenn-Chudnovsky average fragment size expression as the microstructural length of a two-scale damage model. In this construction, the internal length of the model explicitly depends on the local strain rate and the material parameters. The capacity of the new approach to account for the strain-rate sensitivity of the tensile strength is illustrated by comparison of the predicted material and structural responses with experimental data for concrete and ceramic materials.