AA6082 tailor welded blanks (TWBs) produced using laser welding and friction stir welding were studied in this paper. The nominal mechanical properties of welded AA6082 under uniaxial tension conditions were characterised by evaluating the local properties of the base material, heat affected zone (HAZ) and weld zone. Tensile specimens were machined in the way that the weld line lies perpendicular to the loading axis. Three standard-sized specimens containing varying ratios of weld in the gauge region of the specimen were used to determine the size-dependent properties of welded AA6082 in the TWBs. A post-weld strength prediction (PWSP) model, based on the theories of plasticity, has been developed to estimate the post-weld properties of the tailor welded specimens. The model can be used to predict the post-weld yield strength and the global tensile behaviour of welded AA6082 specimens. Good agreements between the modelling and the experimental results have been obtained, with the yield strength deviation less than 6%. It was found that the yield strength increased with increasing dimensions of the tensile specimens. The size-dependent phenomenon was studied and the complex plastic deformation mechanisms have been found to cause the size-dependent phenomenon.