For a telescope operating at a wavelength of 0.5 microns, with an effective aperture size of 13,000 kilometers as in an optical interferometer, you would be able to achieve a resolution of 7 billionths of an arcsecond. To resolve a linear size of 4 centimeters, the maximum distance of the object would be:
4.0 cm / (7 x 10^-9 arcseconds/ 206265 (arcseconds per radian)) = 10^14 centimeters
The distance to Saturn is 1.4 billion kilometers or about this same distance. You would not be able to resolve a human face if it were farther away than the orbit of Saturn. Of course, such an instrument is beyond human engineering and there are MANY technical issues that would limit its actual resolving power well below this mathematical estimate. For more on such supertelescopes and their limitations, see my little essay on The Future of Remote Viewing .