Phase singularities appear in the diffracted far-field of an optical micro-structure if the object is of a sufficient lateral size to induce an appropriate phase delay. We present results that determine the critical dimension of a single phase bar for the generation of dislocations in the far-field. Using scalar theory, an analytical equality is derived that must be met by the structure. Because the size of the object is comparable to the wavelength, rigorous diffraction theory is used to find this feature size for a true object. Once the dislocations appear, their position is related strongly to the geometry of the object. We show theoretically and experimentally by using an interference microscope that, for a single phase bar, the distance between pairwise generated dislocations depends, to a good approximation, linearly on the width of the structure.