This study evaluates the combined roles of particle size and the addition of maleic anhydride polypropylene (PP-g-MAH) to improve 3D printing and mechanical properties of composite filaments based on recycled polypropylene (rPP) and rice husk (RH) obtained as a by-product of rice production. Particle size and PP-g-MAH content are shown to be critical in shaping the filaments' characteristics. Particle size below 250 microns and 5 wt.% of PP-g-MAH resulted in smoother surface finish, improved adhesion between RH particles and the rPP matrix, reduced crystallinity, and enhanced flow properties. Moreover, the study highlighted a significant reduction in the warping effect when RH and PP-g-MAH were incorporated, making the composite filaments more suitable for 3D printing applications. Besides, adding PP-g-MAH improved the printed parts' tensile and flexural properties, promising stronger and more sustainable 3D-printed products. This research contributes to developing eco-friendly 3D printing composite materials aligned with Circular Economy principles.