Hydrogen embrittlement (HE) of steels in the presence of gaseous hydrogen poses a challenge to the safe transportation of hydrogen through pipelines. The use of sub-size specimens machined from coupons extracted from pipes is proposed for monitoring the degradation of material properties over the entire operational life. The objective of this paper is to present a methodology based on the sub-size specimens using a machine developed to operate with a small volume of gaseous hydrogen, up to a pressure of 250 bars, and test control using Edge Tracing (ET) technique. Tensile and toughness mini-specimens extracted from a E355 mod. steel grade are employed for this study. Reliable results are obtained using tensile mini-specimens with 1.2 and 2.4 mm diameters, tested under both air and gaseous H2. Toughness tests are conducted using mini-specimens with a thickness of 5 mm. These tests monitored the Load Line Opening Displacement (LLOD) and the Opening Angle are monitored during the tests through ET technique, allowing for the determination of J−Δa and δ−Δa plots under both air and gaseous H2 conditions. The results uncover the impact of H2 pressure and specimen dimensions on hydrogen embrittlement, leading to the computation of Hydrogen Embrittlement Indexes through various methods. The insights gained from this study establish the necessary foundations for the utilization of mini-specimens in characterizing hydrogen embrittlement.