Current computational simulations on metallic nanowires are largely focused on ultrathin wires with characteristic sizes smaller than 2 nm. The electronic, thermal and optical properties form the bulk of these studies, with investigations of the mechanical properties centred on the breaking force of monatomic chains, and the structural evolution of small nanowires subjected to axial, shear, bending and torsional forces. This study seeks to build on the wealth of current knowledge for computational simulation on the mechanical properties of metallic nanowires. The simulation scale will be upped to 24 000 atoms to study a larger metallic nanowire with a 6 nm characteristic size scale. The commonly studied Au nanowire is studied in conjunction with the rarely examined Pt nanowire. The effects that size and strain rate have on the stretching behaviour of these nanowires are investigated through the simulation of nanowires with three characteristic sizes of 2, 4 and 6 nm, subjected to three distinct strain rates of 4.0 × 10⁸, 4.0 × 10⁹ and 4.0 × 10¹⁰ s⁻¹. The selected strain rates produce three distinct modes of deformation, namely crystalline-ordered deformation, mixed-mode deformation and amorphous-disordered deformation, respectively. The mechanisms behind the observations of these distinct deformation modes are analysed and explained. A Doppler 'red-shift' effect is observed when the nanowires are strained at the highest strain rate of 4.0 × 10¹⁰ s⁻¹. This effect is most pronounced for the nanowire subjected to the largest stretch velocity. As a result, a constrained dynamic free-vibration phenomenon is observed during stretching, which eventually leads to delocalized multiple necking, instead of a single localized neck when it is strained at a lower rate. This unique phenomenon is discussed and future research effort is in the pipeline for a more detailed investigation into metallic nanowires strained at a supersonic velocity.