We develop a picture of graphenic crystallites within disordered carbons that goes beyond the traditional model of graphitic platelets at random orientation. Using large atomistic models containing one million atoms, we redefine the meaning of the quantity La extracted from X-ray diffraction (XRD) patterns. Two complementary approaches are used to measure the size of graphenic crystallites, which are defined as regions of regularly arranged hexagons. Firstly, we calculate the X-ray diffraction pattern directly from the atomistic coordinates of the structure and analyse them following a typical experimental process. Second, the graphenic crystallites are identified from a direct geometrical approach. By mapping the structure directly, we replace the idealised picture of the crystallite with a more realistic representation of the material and provide a well-defined interpretation for $L_a$ measurements of disordered carbon. A key insight is that the size distribution is skewed heavily towards small fragments, with more than 75% of crystallites smaller than half of $L_a$.
