In part 1 of the post he mentions the molecular size of PFAs, and in part 2 he mentions benzene. I think "size" is not the relevant property - it's "does this molecule get imported into cells and alter some component of their activity". While molecular size is related to this, it's more about presence of transporters on the cells, hydrophobicity of the molecules, how the liver handles them (e.g. phase I/II metabolism) etc.
You can contrast this with asbestos, which is dangerous because it mechanically disrupts mucus clearance in the lungs without being imported into cells (also why asbestos + smoke is very dangerous, but asbestos alone probably not nearly as problematic).
Microplastics increase the surface area for degradation in the body, and thus might increase intestinal (and then blood) concentrations of plastic component chemicals or heavy metals they complex with. However, I think it's mainly a phenomenon of degradation in the intestine and then import into the blood stream. Contrast this with "microplastics get into the bloodstream intact". While there might be some of that going on (e.g. phagocytosis by macrophages, eosinophils, etc) I think it's the wrong mental model.
As a bit of reference, remember that your large intestine contains about 1E13 microbes/gram of digesta, and they are 0.2-2um in size. If the intestine let things of that size in at high rate we'd all have sepsis all the time