A quantitative relationship is developed between repulsive intermolecular potential surfaces-and molecular electron density contours, using the helium atom as a probe, For a variety of small neutral molecules and anions, the molecular surface consistent with approach of a helium atom at a temperature of 298 K is identified with the electron density contour at 0.0033 +/- 0.0005 au. This gives a prescription for molecular size and shape that is expected to be useful for cavities in solution and other applications.