Introduction: Growing concerns regarding the reproductive toxicity associated with daily life exposure to micro-/nano-plastics (abbreviated as MNPs) have become increasingly prevalent. In reality, MNPs exposure involves a heterogeneous mixture of MNPs of different sizes rather than a single size. Methods: In this study, an oral exposure mouse model was used to evaluate the effects of MNPs of four size ranges: 25–30 nm, 1–5 µm, 20–27 µm, and 125–150 µm. Adult male C57BL/6 J mice were administered environmentally relevant concentrations of 0.1 mg MNPs/day for 21 days. After that, open field test and computer assisted sperm assessment (CASA) were conducted. Immunohistochemical analyses of organ and cell type localization of MNPs were evaluated. Testicular transcriptome analysis was carried out to understand the molecular mechanisms. Results: Our result showed that MNPs of different size ranges all impaired sperm motility, with a decrease in progressive sperm motility, linearity and straight-line velocity of sperm movement. Alterations did not manifest in animal locomotion, body weight, or sperm count. Noteworthy effects were most pronounced in the smaller MNPs size ranges (25–30 nm and 1–5 µm). Linear regression analysis substantiated a negative correlation between the size of MNPs and sperm curvilinear activity. Immunohistochemical analysis unveiled the intrusions of 1–5 µm MNPs, but not 20–27 µm and 125–150 µm MNPs, into Leydig cells and testicular macrophages. Further testicular transcriptomic analysis revealed perturbations in pathways related to spermatogenesis, oxidative stress, and inflammation. Particularly within the 1–5 µm MNPs group, a heightened perturbation in pathways linked to spermatogenesis and oxidative stress was observed. Conclusions: Our data support the size-dependent impairment of MNPs on sperm functionality, underscoring the pressing need for apprehensions about and interventions against the escalation of environmental micro-/nano-plastics contamination. This urgency is especially pertinent to small-sized MNPs.