Here's one source about the size of microplastics:

The researchers couldn't give a precise breakdown of the particle sizes due to the limitations of the testing methods. It's safe to presume, however, that smaller particles closer to the 700 nanometer limit of the analysis would be easier for the body to take in than larger particles exceeding 100 micrometers.

And red blood cells are 6.2–8.2μm. So I wonder if some kind of sieve could filter anything smaller than 5μm or so. Then again, there's probably a bunch of other stuff in there as well, like white blood cells.

It's too bad that they're in the same range as important things...