The previously reported scattering of He atoms from isolated CO molecules on Cu(001) has been reinterpreted using an exact quantum mechanical model assuming a hard hemisphere on a hard flat surface. From a best fit of all the observed oscillations for each incident energy an effective radius for the hemisphere is determined. The radius increases from a=2.4 to 4.4 Angstrom as the incident energy decreases from 40.3 to 9.4 meV, as expected in view of the soft repulsion and long-range attraction of the true potential.