Naturally existing biological materials have been garning considerable attention as environmentally benign green-nanofactories for the fabrication of diverse nanomaterials, and with desired size and shape distributions. In the present investigation, we report the size and shape controllable biofabrication of silver nanocrystallites using the growth extract of the fungus, Rhizoctonia solani. Influence of various factors such as growth medium; radiation, in the form of sun light; and seeding duration on the production of silver nanoparticles using aqueous 1 mm silver nitrate solution under ambient conditions is presented. Our results demonstrate that these factors can significantly influence the production, size and shape transformation, and the rate of nanoparticles formation. Multiple characterization techniques involving UV-visible and Fourier transform infrared spectroscopy, X-ray diffraction, energy dispersive X-ray spectroscopy and transmission electron microscopy measurements confirmed the production, surface and structural characteristics, purity and crystalline nature of the biosynthesized silver nanoparticles. Our biogenic synthesis process provides a simple, ecologically friendly, cost-effective synthesis route, and most importantly the ability to have control over the size and shape distributions that lends itself for various biomedical and opto-electronic applications.