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This paper investigates the effects of the shear span:depth a/d ratio and effective depth d on the behaviour and shear strength of large reinforced concrete deep and shallow beams. A total of seven large- and medium-sized specimens were tested to failure under two-point symmetric top loading to determine their diagonal cracking and ultimate shear strengths. It was found that the beams' ultimate shear strength reduced sharply when the overall height increased from 500 to 1000 mm. The diagonal cracking strength was, however, not size dependent. The shear strength reduction is related to the a/d ratio: that is, at a critical a/d value, the size effect becomes very pronounced; beyond it, the size effect is not as apparent. After diagonal cracking has occurred, deep beams behave like a tied arch, with uncracked concrete as compression struts and main longitudinal reinforcement as tension ties. A finite element program ‘WCOMD’ was used to supplement the experimental investigation on size effect. Based on test results, finite element modelling (FEM) and a tied arch analogy, the myth of size effect is unravelled.