Phase diagrams in the plane of (the average ionic radius, related to the one-electron bandwidth ) and (the ionic radius variance, measuring the quenched disorder), or bandwidth-disorder phase diagrams, have been established for perovskite manganites, with a three-dimensional (3D) Mn-O network. Here we establish the intrinsic bandwidth-disorder phase diagram of half-doped layered manganites with a two-dimensional (2D) Mn-O network, examining in detail the parent state of the colossal magnetoresistance phenomenon in crystals without ferromagnetic instability. The consequences of the reduced dimensionality, from 3D to 2D, for the order-disorder phenomena in the charge-orbital sectors are also highlighted.