The noninvasive quantification of axonal morphology provides an exciting avenue to gain understanding of the function and structure of the central nervous system. Accurate non-invasive mapping of micron-sized axon radii using commonly applied neuroimaging techniques, i.e., diffusion-weighted MRI, has been bolstered by recent hardware developments. In this work, we present the whole brain characterization of the effective MR axon radius and evaluate the inter- and intra-scanner test-retest repeatability and reproducibility to promote the further development of the effective MR axon radius as a neuroimaging biomarker. We observe a coefficient-of-variability of approximately 10% in the voxelwise estimation of the effective MR radius in the test-retest analysis, but demonstrate that the performance can be improved fourfold using a customized along-tract analyses.