Representative elementary volumes (REVs) and heterogeneity are key concepts in continuum multiphase flow, yet their manifestation from the pore-scale and associated impacts with the flow regime are not well understood. We use a multi-scale experimental and modelling approach to elucidate the role of REVs, hysteresis and heterogeneity in multiphase flow in two distinct water-wetting Bentheimer sandstones. Experimental observations during steady-state drainage & imbibition resolve the pore space at 6µm across a field of view up to 12×12×65mm with simultaneous measurements of differential pressure during fluid flow. The REV for porosity and capillary pressure is ≈2mm3 for both samples, with 5% relative uncertainty. In contrast, due to macroscopic capillary pressure heterogeneities, the REV for saturation varies between 3.4-157.5mm3, dependent on the fractional flow. Accurate knowledge of this REV uncertainty is critical in assessing continuum scale model validity and predictiveness. We validate the Land trapping model directly by predicting observed non-wetting fluid connectivity at multiple imbibition states to within the REV uncertainty for homogeneous media. With this, observed hysteresis in the measured relative permeability is entirely removed when considering the connected saturation, validating conceptual hysteresis models based on connected pathway flow. REV scale heterogeneities in capillary pressure are shown to impact flow and trapping characteristics; when built into 3D continuum scale numerical models with hysteresis we are able to predict the variations in trapping and relative permeability observed between the two samples with different heterogeneity structures. The experimental dataset herein provides an excellent benchmark for future development of continuum scale modelling frameworks.