UV-C lamps in form factors similar to common fluorescents are available from lighting specialty suppliers, and would probably be a better choice for such an application since they offer significantly more UV radiation per dollar than current UV LEDs.
Short of taking things to ridiculous extremes, I’ve no hard & fast answer to the “which bulb at (x) distance” question. In general, the more incident UV energy per area of treated surface, the more potent the effect. There will be some sort of exposure time vs. intensity curve needed to achieve a given level of reduction in viable cells, and it will change somewhat depending on the UV wavelength used. What those numbers are, I don’t know.
Beyond that, things such as the porosity of the treated surface can also have an effect; the UV photon bullets have to hit the viral cells to do damage, so if the surface in question provides lots of foxholes in which they can hide during bombardment, the treatment will be less effective.