<p>This study investigates the mechanism of the size effect on the tensile properties of compacted clay. Threepoint bending tests are performed to measure the tensile strength of compacted clay beams of different sizes. Digital image correlation is adopted to determine several mesoscopic tensile evaluation indicators. The results show that the load–displacement curves for specimens of different sizes can be divided into four stages: compression, linear elastic deformation, plastic deformation, and dramatic decrease. The critical opening displacement at the crack initiation point increases with increasing specimen size, but the rate of increase gradually slows. The peak load strain is negatively correlated with the specimen size, exhibiting a linear relationship. Furthermore, the tensile strength shows a significant size effect that decreases with increasing specimen size. Based on Bažant–Xi theory, a predictive model for the tensile strength of compacted clay is established. Finally, equations relating the tensile strength, specimen size, and strain of compacted clay are proposed, and the factors influencing the tensile strength of compacted clay in laboratory tests are analyzed.</p>