An experimental and numerical tomographic study on the influence of lateral beam coherence and limited detector resolution on the size of voids in copper is presented. Several scans of the same sample were performed at ID15A of ESRF under pink beam conditions and different sample-detector distances. The tomographic images were segmented using a gradient based method and then the same voids appearing in subsequent volumes were identified through image-correlation. It was found that the segmented void size is smaller at short detector distances, but it saturates at values of about 160-200 mm. Simulations show that the phase contrast available at larger distances enhances the intensity gradient at void-matrix interfaces leading to a proper segmentation if gradient based algorithms are used. For distances involved in the present experiment the true size of voids with diameters in the 4.7-12 mu m range is not altered by phase contrast effects. The smaller apparent void size (by about 15-20%) obtained at short distances (20-50 mm) is a result of low signal to noise ratio of the corresponding reconstructions.