The specimen size requirements in toughness qualification are discussed on the basis of statistical local fracture criterion approach. The Weibull stress criterion for cleavage fracture is applied to the constraint loss field at the crack tip described with the toughness scaling model (TSM). Proposed new model (WSSM) demonstrates that the TSM is a particular case fora material with a particular combination of the Weibull shape parameter, m and the strain hardening exponent, n . The correlation between TSM and WSSM were also confirmed with 2 dimensional and 3 dimensional finite element analysis. The 3 dimensional analysis with J_p -δ for the stress intensity suggested the less constraint loss effect rather than 2 dimensional analysis in spite of the strong dependency of the constraint loss effect on Weibull shape parameter, m . The 3 dimensional analysis also suggested that the thickness effect on toughness mainly resulted from the statistical volume effect. The values of the hardening exponent, n and Weibull shape parameter, m , that are the dominant factors for the specimen size requirement suggested from WSSM, were investigated from the experimental results for the practical constructive steels. These results demonstrated that the specimen size requirements prescribed in ASTM could be too conservative.