© 2020, The Korean Society of Mechanical Engineers and Springer-Verlag GmbH Germany, part of Springer Nature.In this paper, the size effect in unidirectional (UD) laminated glass-fibre reinforced plastic (GFRP) composites subjected to quasi-static bending loading was investigated: the sensitivity of a specimen’s mechanical behaviour and failure mechanism to its geometry was studied. Composite beams with different numbers of 0° unidirectional plies were tested and their post-deformation structures were analysed microscopically. In the subsequent simulations, the intraply damage was modelled using continuum damage mechanics, implemented as a user-defined VUMAT subroutine in ABAQUS/Explicit, while cohesive zone elements were employed to characterize the delamination between different plies. It was observed that the flexural failure triggered the multiple delaminations; their location was studied. The influence of the size effect on the bending response of the UD composite beams was analysed in depth. The findings of the current study can be used to design modern structures made of composite materials.