The utilization of silicon nanostructures, from quantum dots to nanowires, for photovoltaic applications depends on understanding the effect of their physical structure on their optical absorption properties. In this work, we perform TDDFT calculations to study the length dependent optical absorption in pristine and doped silicon nanostructures. Our main findings are that: (i) The oscillator strength as a function of length is quadratic at small lengths, and then increases linearly. (ii) The exciton binding energy is seen to decrease by approximately 45% from 0.67 eV to about 0.3 eV with length increase, for a nanostructure with a cross-section diameter of approximately 12 A. (iii) Doping and codoping with P, B, and Zr have the potential to cause the optical absorption to change from UV to the visible spectrum. The findings of this investigation demonstrate the potential to tailor silicon nanostructures for photovoltaic and optoelectronic applications. (C) 2015 Elsevier Ltd. All rights reserved.