Abstract: The problems of tailings storage and high-stress conditions in deep mining have emerged as critical factors that limit the security, efficiency, and sustainability of such mines. This study explores the potential to utilize tungsten tailings to create cementitious backfill (CTB) materials and investigates the macroscopic strength features and microscopic damage evolution mechanisms of different-sized CTBs with varying dosages of hydroxypropyl methyl cellulose (HPMC). Specimens with bottom diameters of 50 mm, 75 mm, and 100 mm are combined with HPMC dosages of 0, 0.15wt%, 0.25wt%, and 0.35wt%. A diameter/height ratio of 1:2 is maintained for all CTB specimens. The experimental results show that as the HPMC dosage is increased from 0 to 0.35wt%, the uniaxial compressive strength (UCS) of the CTBs decreases significantly in a linear manner. The 75 mm × 150 mm CTB specimen exhibits relatively high plasticity and toughness, with good plastic deformation and energy absorption capabilities, indicating significant size effects. HPMC introduces connected bubbles during the CTB pouring process, but it exhibits anti-segregation and anti-bleeding characteristics, thus reducing tailing settling. The hydration reaction of the CTB doped with HPMC is more uniform, and the Ca/Si atomic ratio dispersion at different sites is smaller. The three CTB sizes all exhibit combined tensile and shear failure, with the 75 mm × 150 mm specimen exhibiting macroscopic tensile cracks and relatively few shear cracks. At the micro-scale, excessive ettringite and hydrated calcium silicate are interwoven and fuse, and the tungsten tailings are tightly wrapped. These results provide valuable data and notional insights for optimizing the fluidity of the backfill, and elucidate the strength and damage evolution of solidified materials during filling and extraction. This study contributes to the advancement of green, economical, safe, and sustainable mining practices.