Abstract: As the size of fiber-reinforced polymer (FRP) laminated structures increases, the probability of internal defects rises, leading to a reduction in fatigue performance and fatigue life. Therefore, the size effect must be carefully considered during the design of FRP structures. In sight of this, it is crucial to investigate both the qualitative and quantitative impact of size effect on the fatigue life of FRP structures. This paper investigates the bending fatigue performance and fatigue life of glass fiber-reinforced polymer (GFRP) laminates with five different sizes (scaled proportionally as 1∶1.5∶2∶2.5∶3 for length, width, and thickness) and monitors the cumulative damage during fatigue using ultrasonic C-scan testing. The results indicate that, compared to the baseline specimen (size ratio of 1), the bending fatigue life of the other four sizes of GFRP laminated specimens decreased by 29.26%, 42.24%, 61.14%, and 71.14%, respectively. Failure of bending fatigue typically occurs at the clamping ends, manifesting as brittle fracture of the cross-section with a zigzag appearance at the fracture surface. Ultrasonic C-scan testing shows that larger-size specimens experience more cumulative fatigue damage under the same number of cycles. Furthermore, a quantitative analysis of size effects was conducted. A fatigue life prediction model for different sizes of GFRP laminates is proposed based on Weibull analysis, enabling quantitative assessment of the fatigue life for GFRP specimens of various sizes. This paper provides valuable references and theoretical support for the design of FRP components, such as FRP wind turbine blades which are primarily subject to bending fatigue failure.