Size of metal nanoparticles (NPs) plays decisive roles in metal-catalyzed heterogeneous reactions, due to the drastic variation of the geometric and electronic properties of metal NPs with size. Along with the development of controlled catalyst synthesis, tremendous efforts have been devoted to understanding the nature of the particle size effect. In particular, identification of the individual roles of size-dependent geometric and electronic effects on metal-catalyzed reactions is essential, but remains challenging since they are tightly hybridized together with particle size variation. In this review, we first discuss the fundamentals of the size-dependent geometric and electronic properties of metal NPs and their crucial roles in catalysis in general. Then we summarize the previous representative studies of the particle size effect, metal-by-metal, in heterogeneous catalysis. We highlighted the extension of particle size effect to ultrafine cluster and single-atom catalysts, the new frontiers in heterogeneous catalysis. In the followings, we further introduce the recent advances in disentangling the size-dependent geometric and electronic effects and unveiling their individual contributions to catalysis. Finally, we discuss the challenges and perspectives of investigations on the size effect in metal catalysis for the future studies. Mr. Hengwei Wang (left) received his B.Sc. degree from School of Chemistry and Materials Science, University of Science and Technology of China (USTC) in 2015. Then he Joined Professor Junling Lu's research group at USTC to pursue his doctorate in Physical Chemistry. His research focuses on atomically-precise design and synthesis of metal nanocatalysts for heterogeneous catalysis and unravelling their structure-reactivity relations at atomic scale. Prof. Junling Lu (right) received his Ph.D. degree from Institute of Physics, Chinese Academy of Sciences under the supervision of Prof. Hong-Jun Gao in 2007. During his Ph.D. studies, he visited Prof. Hans-Joachim Freund's group at Chemical Physics Department, Fritz-Haber-Institute, Max Planck Society as an exchange student in 2004-2006. After graduation, he spent three years in Prof. Peter C Stair's group at Northwestern University and then about two and a half years in Dr. Jeffrey W. Elam's group at Argonne National Laboratory as a Postdoc. In March 2013, he joined USTC as a Professor. His current research interest is atomically-precise design of a new generation of advanced catalysts through a combined wet-chemistry and atomic layer deposition (ALD) approach.