Thallium-based high-temperature superconductors, particularly the Tl-2223 phase, exhibit strong structural sensitivity to cation substitution. While most previous studies have focused on partial Ba-site substitution, the effect of complete Ba-site replacement remains insufficiently explored. In this work, the atomic size effect on the structural properties of Tl-2223 was investigated by fully substituting Ba²⁺ with smaller cations, Ca²⁺ and Mg²⁺, introducing negative chemical pressure into the lattice. Polycrystalline Tl₂Ba₂Ca₂Cu₃O₁₀, Tl₂Ca₄Cu₃O₁₀, and Tl₂Mg₂Ca₂Cu₃O₁₀ samples were synthesized using the solid-state reaction method and characterized using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The XRD results revealed systematic peak shifts toward higher diffraction angles, accompanied by lattice distortion, increased microstrain, and higher dislocation density with decreasing ionic radius. These findings highlight the strong influence of atomic size on the structural framework of Tl-2223 and provide a basis for future studies on superconducting properties.