Abstract: To meet the demand for fracture toughness evaluation of CLF-1 steel used in key structural components of fusion reactors under limited sampling conditions after neutron irradiation, a geometrically scaled miniature single-edge bend (MSEB) specimen was designed, and a corresponding fracture toughness testing procedure was established. The objective of this work is to determine whether the MSEB specimen can effectively predict the fracture toughness of standard single-edge bend (SEB) specimens while maintaining a high crack-tip constraint condition. Fracture tests were conducted on MSEB specimens with different crack sizes and on standard SEB specimens at room temperature and 300 °C. The J-resistance (J-Δa) curves and the conditional crack initiation toughness (J_Q) were obtained. Combined with elastic–plastic finite element analyses, the local crack-tip stress states of specimens with different sizes were compared, and the consistency of crack-tip constraint between MSEB and standard SEB specimens was verified. Furthermore, a crack-tip constraint quantification method based on the characteristic strain energy density (w_c) was introduced to systematically evaluate the effects of specimen size and temperature on fracture toughness. A quantitative linear relationship between w_c and J_Q was established, enabling a unified characterization of the local constraint parameter and the global fracture toughness. In addition, the J_Q value of the standard SEB specimen was predicted by the intersection method of the crack-driving force curve and the fracture failure curve, and the prediction was validated against the experimental result. The results show that, at room temperature, the J-Δa curves and J_Q values of MSEB specimens with different crack sizes are highly consistent with those of standard SEB specimens, indicating a weak size effect. At 300 °C, however, the scatter of fracture toughness for MSEB specimens increases significantly, and the J-Δa curves and J_Q values of standard SEB specimens are markedly higher than those of MSEB specimens. A clear linear relationship between w_c and J_Q is observed at both temperatures. The predicted J_Q values of standard SEB specimens agree well with the experimental averages, with relative errors of 0.78% and 10.85%, respectively. These results demonstrate that the MSEB specimen, combined with the w_c-based constraint quantification method, can effectively predict the standard-size fracture toughness of CLF-1 steel and provides a reliable technical approach for the fracture toughness evaluation of irradiated materials using small specimens.