Excess of cadmium (Cd) contamination in soils poses a significant challenge to global agricultural crop production. The potential of silicon (Si) nanoparticles (NPs) and biogenic Si sources of different sizes to mitigate Cd stress in wheat remains largely unexplored. A pot experiment was conducted to evaluate the effectiveness of various sizes of biogenic Si sources—namely rice husk biochar (RHB), sugarcane bagasse (SB), and rice straw (RS)—categorized by particle size (<150 μm, 150–250 μm, and 250–1680 μm, denoted as S1, S2, and S3, respectively), and Si NPs on wheat growth and Cd accumulation. The application of RHB (<150 μm) significantly increased plant height (105%), spike length (94%), shoot dry weight (93%), root dry weight (83%), and grain weight (89%) compared to the control. RHBS1 demonstrated the greatest reduction in the bioavailable fraction of soil Cd (-74%), as well as Cd levels in roots (-85%), shoots (-93%), and grains (-97%) compared to the contaminated control. Additionally, RHB (<150 μm) showed the highest increase in antioxidant enzyme activities compared to the Cd-spiked treatment. Biogenic Si sources have the potential to mitigate Cd stress in wheat plants. Further research is required to understand the underlying mechanisms and to investigate the effectiveness of different biogenic Si sources and particle sizes in alleviating Cd stress in crops.