<p>Difficulty in accurately measuring uniaxial compressive strength (UCS) under high sand-cement ratios due to press range limitations. To facilitate the computation of the UCS value of the cemented paste backfill (CPB) under the condition of high sand-cement ratio, and to analyze the size effect of the CPB with high sand-cement ratio under varied height/width ratio, the CPB specimens with sand-cement ratios of 8:1–24:1 and height/width ratio of 0.5, 1, and 2 were prepared. Through response surface experiments and finite element analysis, the influence of three height/width ratio on the CPB strength was explored, and the failure characteristics and size effect mechanisms of CPB with various height/width ratio were analyzed. The results show that the UCS of the CPB decreases with the increase of the height/width ratio under the same mass concentration and sand-cement ratio. When the height/width ratio is 0.5 and 1, the splitting failure is the main factor, when the height/width ratio is 2, the shearing failure is the main factor. The three-dimensional stress caused by the end effect of the CPB is the main reason for the high strength when the height/width ratio is 0.5. Due to the one-dimensional stress zone generated in the middle of the CPB with a height/width ratio of 2 in the laboratory, its strength value is closer to the actual UCS value of the CPB. The UCS values of the CPB with height/width ratio of 0.5 and 1 were higher than the actual UCS values in the laboratory. The rationality of the theoretical analysis was verified by the ABAQUS analysis results. This study reveals the size effect mechanism and proposes a conversion model for different specimen geometries.</p>