The Système International unit of mass is the kilogram. The present definition of the kg is based on a prototype dating back to 1880s. New approaches to define the unit of mass are being investigated. Avogadro Project uses 10 cm diameter single crystal silicon spheres. The technique commonly observed to measure the radius of the silicon sphere is optical interferometry. Here, we propose an alternate method of measuring the diameter of the single crystal silicon sphere using near-infrared spectroscopy. We demonstrate our approach by numerically simulating the electromagnetic coupling of a silicon microdisk of radius 5 µm to an optical waveguide of width 0.5 µm, thereby approximating the coupling of a microsphere to a rectangular optical waveguide. It might be possible to have a precise technique for determining the radius of the sphere, which can be used for the definition of the kilogram.