Abstract: To investigate the influence of aggregate particle size on the compressive performance and size effect of lightweight aggregate concrete, a mesoscopic random aggregate model incorporating lightweight aggregates, mortar, and interfacial transition zones was established. This model was used to simulate the uniaxial compressive behavior of lightweight aggregate concrete with varying aggregate particle sizes and specimen dimensions. Results indicated that under uniaxial compression, the failure mode of lightweight aggregate concrete was minimally affected by aggregate particle size, specimen size, or aggregate distribution; however, its compressive strength exhibited a distinct size effect. An increase in aggregate particle size reduced the compressive strength of lightweight aggregate concrete, and larger specimen sizes enhanced sensitivity to changes in aggregate particle size. Conversely, a decrease in aggregate particle size weakened the compressive size effect of lightweight aggregate concrete.