Size effect of length is one of important factors to adjust flexural strength of dimension lumber.The dependence between the flexural strength and the size of Chinese fir dimension lumber was investigated.206 dimension lumbers were sorted into three test groups in terms of dynamical modulus of elasticity.Three test spans(135,162 and 207 cm) were selected and flexural property was tested.The length size effects of 5~(th) and 50~(th) percentiles of flexural strength were estimated by the slope method and the ...