This study concerns the computational assessment of the failure modes of hybrid beams made up of prefabricated steel truss encased within a concrete block and named hybrid steel trussed concrete beams (HSTCBs). The considered failure mode is under three-point bending and the adopted approach is developed by means of finite-element (FE) simulations of beams of different scaled sizes, whose design corresponds to current industrial practice. The FE model is based on well-established constitutive relations of both concrete and steel. In particular it employs a continuum damage model for concrete (concrete damaged plasticity model) and a classical elastic-plastic multi-linear behavior for steel. As regards contact at the interface, two different approaches are used and compared: a simplified contact condition able to simulate perfect bond and a non-linear cohesive interaction capable of reproducing the progressive degradation of the initial contact condition. Furthermore, models are generated also employing finite elements of different type and order and, therefore, the computational efforts required by each model are compared in order to evaluate the efficacy of the simplest modelling type able to retain the salient features of the structural mechanism. Existing experimental data are used for validating the numerical results. The simulations on specimens of three different sizes show that, in the absence of three-dimensional geometrical similarity, the small-size beam exhibits shear failure while the large-size beams attain a more ductile failure under flexure. The numerical results allow observing a transition between different failure modes, which indicates the importance of employing a robust three-dimensional FE model for design extrapolation of HSTCBs across different sizes and geometries.