Radiotheranostic agents combine targeted therapeutics and diagnostic imaging, offering more efficient and personalized treatment strategies. Positron emission tomography provides high sensitivity and quantitative accuracy; however, its integration with α-particle radiotherapy remains limited by the lack of chelators capable of stabilizing radionuclides with widely different ionic radii. 89Zr and 225Ac are promising radionuclides for PET imaging and targeted α-therapy, respectively, but require robust and versatile chelation strategies. Herein, we report stable coordination using the 18-membered macrocyclic dual size-selective chelator py2-macrodipa with [89Zr]Zr4+and [225Ac]Ac3+ ions. The chelator demonstrated efficient radiolabeling with both 89Zr and 225Ac, exhibiting 89Zr labeling efficiency comparable to the state-of-the-art chelator DFO at micromolar concentrations....