Plastic pollution has been the focus of attention in recent years, with research on micro- & nano-plastics increasing exponentially. Nevertheless, several challenges associated with the small particles analysis and plastic debris assessment are still faced globally. While conducting a thorough literature review, a clear size knowledge gap between large plastics and micro- & nano-plastics is identified. This gap is evident for mesoplastics, particularly in the aims and methods to assess middle-size range plastics. Large plastic debris are likely to break into smaller fragments, and micro- & nano-plastics are continuously generated from weathered fragments due to multiple environmental stressors. Therefore, small plastic fragments (0.5–5 cm) can act as a bridge between large and micro- & nano-plastics. Here, we highlight the importance of small plastic fragment investigation to address knowledge gaps, which can be used to overcome larger management and policy challenges, while providing new insights on the plastic pollution research field.