Diffusion-weighted MRI (dMRI) is a formidable technique for non-invasively characterizing brain microstructure. Biophysical modelling is often necessary to gain specificity to cellular structure. However, designing sensible biophysical models and appropriate dMRI acquisitions is challenging, especially for gray matter (GM), as little is known about typical values of relevant features of brain-cell morphology contributing to dMRI signal. This study addressed this unmet need: we analysed ~3,500 cells from mouse, rat, monkey and human brains to determine statistical distributions of 13 morphological features relevant to GM microstructure modelling. Illustrative examples demonstrate how this study can inform biophysical modelling.