By modulating the A-site cational size mismatch we introduce different A-site disorder strengths in large-bandwidth manganites such as La0.55Sr0.45MnO3, which are regarded as the typical ferromagnetic double exchange (DE) systems due to the relative large A-site cational mean radius ( = 1.23 angstrom for La0.55Sr0.45MnO3). It is revealed that the increasing A-site disorder doesn't induce remarkable macroscopic change of the crystallographic structure and lattice constants, but causes notable change of the magnetotransport properties by suppressing the DE interaction and the ferromagnetism. For small A-site disorder, the long-range ferromagnetism is broken and replaced with the short-range antiferromagnetic regions embedded in the ferromagnetic matrix. The high A-site disorder will eventually lead to melting of the long-range ferromagnetic order into the short-range magnetically ordered clusters. And the enhanced magnetoresistant effects were found in the samples with intermediate disorder. (C) 2010 Elsevier B.V. All rights reserved.