The influence of the geometry of a longitudinal hollow cathode discharge (HCD) excited in a Cu cathode and He–Ar mixture is studied experimentally and theoretically. Special attention is devoted to the optimization of the HC length to obtain a stable and uniform laser medium with a high excitation efficiency. The influence of the cathode length is demonstrated experimentally by the behaviour of the 780.8 nm Cu ion line. The dependence of the laser power and gain as a function of the cathode length segments is measured. The Plasimo modelling platform is used to construct a model allowing more profound studies of the plasma processes and plasma behaviour under different conditions. Calculations at different cathode lengths are made and typical results such as spatial potential and plasma density distributions are presented and discussed. It is demonstrated that when the cathode length is increased the plasma density at the centre of the cathode decreases and the discharge tends to separate into two independent parts causing axial non-uniformity and reducing the discharge efficiency. The results also suggest that there exist a lower limit of the cathode length. Below this limit an inversion of the axial electric field occurs, which can be regarded as a transition between the conventional and high-voltage HCD under the conditions under study.