Tensile strength constitutive laws for fibre reinforced concrete (FRC) are commonly defined through the parameters of flexural tests conducted on standard prismatic specimens. However, there are no specific criteria to determine such parameters using small specimens that could simplify the testing procedure and provide more representative results of slender structural FRC elements. In this line, the influence of size effect becomes an issue particularly relevant during the characterisation stage given that the residual strength decreases while increasing the size of the element. The objective of this document is to propose a methodology to obtain the parameters of the constitutive law using small specimens. For this, FRC residual strength was determined through three-point bending tests on prismatic notched beams of 40 × 40 × 160, 100 × 100 × 400 and 150 × 150 × 600 mm. An analytical model based on sectional analyses aimed at reproducing the flexural strength of FRC was used to assess the results of the alternative methodology to determine the parameters for the constitutive law. The results show that an approach based on the rotation instead of the crack opening as the reference parameter to estimate the stresses for the constitutive law leads to results less influenced by the size effect when designing small elements.