Radioisotopes of metallic elements, or radiometals, are widely employed in both therapeutic and diagnostic nuclear medicine. For this application, chelators that efficiently bind the radiometal of interest and form a stable metal-ligand complex with it are required. Toward the development of new chelators for nuclear medicine, we recently reported a novel class of 18-membered macrocyclic chelators that is characterized by their ability to form stable complexes with both large and small rare-earth metals (Ln), a property referred to as dual size selectivity. A specific chelator in this class called py-macrodipa, which contains one pyridyl group within its macrocyclic core, was established as a promising candidate for La, Bi, and Sc chelation. Building upon this prior work, here we report the synthesis and characterization of a new chelator called py-macrodipa with two pyridyl units fused into the macrocyclic backbone. Its coordination chemistry with the Ln series was investigated by NMR spectroscopy, X-ray crystallography, density functional theory (DFT) calculations, analytical titrations, and transchelation assays. These studies reveal that py-macrodipa retains the expected dual size selectivity and possesses an enhanced thermodynamic affinity for all Ln compared to py-macrodipa. By contrast, the kinetic stability of Ln complexes with py-macrodipa is only improved for the light, large Ln ions. Based upon these observations, we further assessed the suitability of py-macrodipa for use with Ac, a large radiometal with valuable properties for targeted α therapy. Radiolabeling and stability studies revealed py-macrodipa to efficiently incorporate Ac and to form a complex that is inert in human serum over 3 weeks. Although py-macrodipa does not surpass the state-of-the-art chelator macropa for Ac chelation, it does provide another effective Ac chelator. These studies shed light on the fundamental coordination chemistry of the Ln series and may inspire future chelator design efforts.