How do microplastic particle sizes influence bioaccumulation and toxicity in freshwater ecosystems?

Eating microplastics is completely different from eating flour. They can "penetrate" from the intestines into the blood and tissues. Large particles are usually excreted after consumption, while small particles remain. Small particles have a larger specific surface area, and more heavy metals and pathogens are adsorbed on their surface, which can lead to various diseases.

 Microplastic particle size  plays an important role  in their  bioaccumulation and toxicity in freshwater ecosystems. The particle sizes influences how these microplastic particles move through freshwater ecosystems, how organisms take them up, and how harmful they can be. Smaller microplastics are easily ingested by many freshwater organisms due to their resemblance to food. Due to their small size  some particles can cross biological barriers and persist in tissues, thus increasing retention and trophic transfer. In contrast, larger microplastics are consumed by fewer species and are more easily excreted, resulting in lower bioaccumulation. On the other hand, the  particle size strongly influences toxicity.  The smaller the  particles, the higher  surface-area-to-volume ratio. This allows them to adsorb more pollutants and also host microbial communities. When ingested, these contaminants can desorb and increase chemical exposure inside organisms. Very fine microplastic particles  or the ones at nanometric scale can induce physical and cellular stress, such as inflammation, oxidative stress, and damage to tissues, because they can interact directly with cells and even enter organs while larger particles can only cause physical effects and are generally less toxic at the cellular level. The bioaccumulation potential and toxicity generally increase with the microplastic particle size decrease. 

Bioaccumulation of microplastics may be a result of their ingestion along with food or water by aquatic organisms.Then the smaller particles cross the epithelial layer into the blood stream, while the bigger particles are passed out into the water during egestion. This means that there is accumulation of these microplastic particles downstream, with an increasing concentration effect along the food chain.
The toxicity of these microplastic particles may be as a result of
-Some carcinogenic chemical         substances associated with their   manufacture
-These particles interfering with the   normal absorption process - over time   can 'clog' the epithelial layer
-These particles can also compete   with  the nutrients particles during   absorption, resulting in an organism   being deprived of essential nutrients
-The changing PH conditions in the   different parts of the GIT, probably   affect the physical and chemical   state.  of the microplastic particles,   which can translate into toxicity 

Larger microplastics tend to remain in the gut and mainly cause physical stress, such as blockage and reduced nutrition. Smaller microplastics can cross epithelial barriers, accumulate in organs and induce oxidative stress and inflammation. Whereas, nanoplastics penetrate cells, reach sensitive tissues such as the brain and gonads, alter gene expression and cause systemic toxicity.

Larger particles are less likely to be ingested, and if ingested are less likely to be absorbed into the tissues and more likely to work their way down the digestive tract and and be egested.