In recent years, the toxicity of microplastics (MPs) in combination with heavy metals, particularly the influence of varying microplastic sizes on their toxic effects, has attracted widespread attention. In this study, red swamp crayfish (Procambarus clarkia) were exposed to MPs of two particle sizes (S-MPs: 5 μm, 1 mg/L
and L-MPs: 100 μm, 1 mg/L) and Cu (5 mg/L) individually or in combination for 96 h. The accumulation patterns of MPs were as follows: gills >
intestines >
hepatopancreas >
muscles. Moreover, the accumulation pattern of Cu was as follows: intestines >
gills >
hepatopancreas >
muscle. Additionally, S-MPs and L-MPs enhanced Cu accumulation, with the highest levels observed in the S-MPs+Cu-treated group. Histopathological analysis showed that the combined exposure led to greater hepatopancreatic damage. Assessment of antioxidant enzymes showed decreased activities of superoxide dismutase, catalase, and glutathione among the different treatments, except for malondialdehyde, which was elevated compared to the control group. In the S-MPs+Cu-treated group, the expression levels of Cu homeostasis genes (MTF-1, ATP2, Atox1, MT) were significantly lower than those in the Cu treated group. This study provides a valuable reference for studying the combined toxic effects of MPs with varying particle sizes on heavy metals.