Strike-slip faults are generally described as continuous structures, while they are actually formed of successive segments separated by geometrical complexities. Although this along-strike segmentation is known to affect the overall dynamics of earthquakes, the physical processes governing the scale of this segmentation remain unclear. Here, we use analogue models to investigate the structural development of strike-slip faults and the physical parameters controlling segmentation. We show that the length of fault segments is regular along strike and scales linearly with the thickness of the brittle material. Variations of the rheological properties only have minor effects on the scaling relationship. Ratios between the segment length and the brittle material thickness are similar for coseismic ruptures and sandbox experiments. This supports a model where crustal seismogenic thickness controls fault geometry. Finally, we show that the geometrical complexity acquired during strike-slip fault formation withstands cumulative displacement. Thus, the inherited complexity impedes the formation of an ever-straighter fault, and might control the length of earthquake ruptures.