An image processing workflow is presented for the characterization of pore and grain size distributions in porous geological samples from X-ray microcomputed tomography (μCT) and scanning electron microscopy (SEM) images. The pore and grain size distributions of five sandstone samples including Berea, Buff Berea, Nugget, Castlegate, and Bentheimer, and one carbonate sample, Indiana limestone, are extracted using the proposed workflow. Two-dimensional size distributions acquired from SEM images were found to be biased toward smaller sizes misrepresenting the actual 3D distributions. Stereological techniques unfolded the measured 2D size distributions from SEM images to 3D distributions comparable with μCT results. While larger pores and grains can easily be detected from μCT and SEM images, the quantification of small-scale heterogeneities is severely influenced by their limits of resolution. We show that microstructural details resolved by SEM can significantly impact the pore and grain size distributions in sandstone and carbonate rock samples. For example, SEM-resolved microporosities in Indiana limestone result in bimodal distributions of pore and grain sizes, whereas μCT observations exhibit unimodal distributions. The acquired images and processed results are openly available and may be used by researchers investigating image processing, magnetic resonance relaxation or fluid flow simulations in natural rocks. The proposed methodology can be implemented to process μCT and SEM images of natural rocks as well as other types of porous materials. •Image processing workflow computes pore/grain size distributions from images.•Stereological unfolding of pore/grain size from SEM agrees with that of μCT images.•SEM resolves microstructural details that affect pore and grain size distributions.•The agreement between μCT and SEM results was consistent in six sedimentary rocks.•A dataset of μCT and SEM images of six sedimentary rock samples made public.