<p>In split Hopkinson pressure bar (SHPB) tests, the size effects play a crucial role in influencing dynamic mechanical properties of rock-like materials. To explore the influence of specimen size and bar diameter on the dynamic mechanical characteristics of sandstone, dynamic compression experiments were conducted on specimens of varying sizes using a Φ50 mm and a Φ100 mm SHPB device. The results showed that specimens exhibited different test outcomes under the impact of bars with different diameters. In addition, in order to obtain the influence of size effects on axial inertia effects, the concept of dimensionless stress difference was proposed, which can be obtained by installing strain gauges at both ends of specimens. Dimensionless stress difference-time curves can be divided into an oscillation zone and a stable descent zone. Research indicates that the duration of the oscillation zone is largely constrained by the diameter of the bar and the intensity of the impact, and is relatively less affected by the length of specimens. The results can provide theoretical references for designing specimen sizes of rock-like materials in SHPB tests at the macro level, and these findings provide valuable insights for optimizing impact-resistant designs in rock-based engineering structures.</p>