This paper examines the effects of sample size and boundary conditions on the necking inception and development in dynamically stretched steel specimens. For that task, a coordinated systematic experimental-numerical work on the dynamic tensile test has been conducted. Experiments were performed using a tensile Kolsky apparatus for impact velocities ranging from 10 to 40 m/s. Three different sample-gauge lengths - 7, 30 and 50 mm - were considered for which the cross section diameter is 3.4 mm. The experiments revealed that the specimens' ductility to fracture depends on strain rate and sample length. Furthermore it was observed that, for those specimens having gauge lengths of 30 and 50 mm, the necking location varies with impact velocity. Numerical simulations of the dynamic tensile tests were carried out in order to characterize the dynamics of neck inception and development. For each specimen calculated, three types of boundary conditions were used, all of which match the experimentally measured strain-rate. It was pointed out that, while boundary conditions hardly affect the calculated stress-strain characteristics, they strongly affect the wave propagation dynamics in the specimen thus dictating the necking location. © 2013 Elsevier Ltd. All rights reserved.