A commonly encountered problem in fracture toughness testing is the question regarding the valid specimen size. Different testing standards have different requirements for the different minimum specimen dimensions. These requirements are, however, not always realistic nor correct. In this paper the effect of ligament size on cleavage fracture toughness in the elastic-plastic regime is considered. The consideration is performed within the framework of a statistical cleavage fracture model (WST). A new specimen ligament size requirement for elastic-plastic cleavage fracture toughness testing is presented, together with a correction function to validate invalid test results.