The formation of macropores by anodization of low-doped p-Si in HF electrolyte has been investigated quantitatively. As anodization proceeds, structures of increasing characteristic size are formed, then a steady state is reached, where macropores grow parallel. The intermediate regime is well understood on the basis of a linear stability approach, incorporating the known physics and chemistry of the Si/electrolyte interface: semiconductor space charge and interface reaction velocity. The characteristic size of the macropores and their dependence on Si doping and electrolyte resistivity and composition are quantitatively accounted for after realizing that parallel growth is strongly favored by the channeling of the current in the macropores. Below a critical resistivity, no macropores are observed. It is shown, through a numerical simulation, that this change of behavior results from a loss of the insulating character of the walls, due to effects of disorder in a depletion layer when doping increases.