What would the most ideal combination of methods be to ensure confidence in both size and chemical ID?

I would use uFTIR/uRaman for particle counting and polymer identification; py-GC/MS would be used as a confirmation (for polymer) but also for additive identification. And, initially, visual microscopy for morphological (and color) classification. 

In my view, one should not be prescriptive in this respect; there are likely to be numerous combinations of techniques that can provide reliable quantitative and qualitative information. The "acid test" is whether whatever combination is employed can demonstrate its accuracy and reproducibility - e.g. via replicate analysis of one or more of standard/certified reference materials, appropriate matrix spikes, and/or successful participation in an interlaboratory comparison.

One of the most effective method for ensuring confidence in both particle size measurement and chemical identification of plastics in biological tissues, involves  complex complementary  techniques that can  provide  results about size, polymer identification and also information about the minimal interference that can come from complex biological matrices. A possible combination should include first, the extraction which should be performed with the aid of enzymatic digestion or using some mild chemical treatments, ideally combined with density separation to try and  isolate the plastics from the organic tissues while trying to preserve the polymer types and reducing the interferences that may arise from the matrix effects. Next, a particle imaging and also size measurement should be carried out with the aid of optical microscopy for particles larger than 20 µm, or scanning electron microscopy for smaller particles, thus ensuring a good evaluation of size, shape, and count. If there are polymers involved, a combination of μFTIR imaging, Raman spectroscopy, and high-resolution techniques such as nanoFTIR or OPTIR that could provide a broad spectral coverage and also improved resolution of overlapping particle types should be used for for polymer identification. Particularly for particles too small for imaging, pyrolysis-GC/MS analysis should be used to confirm polymer identity.

Polarisation wave or electron microscopy can often support the analysis of plastic particles when the type of plastic is known. However, microscopy alone cannot provide visual evidence of plastic presence in complex samples (like tissues) without chemical confirmation of the same particles.  Distinguishing MNPs from organic background in such complex matrices is extremely challenging due to their low density and the potential for visual misidentification. Without chemical confirmation through techniques like AFM-FTIR or µ-Raman spectroscopy, it is difficult to conclusively identify the plastics assumed to be present. Raman spectroscopy offers for instance high spatial resolution and is capable of identifying microplastics as small as a few micrometres, making it a valuable tool for such analyses. Therefore, relying solely on visual evidence without corroborating chemical analysis may lead to inaccurate conclusions regarding the presence of plastics in biological tissues.
An ideal combination thus does not exist and one should use combinations of methods, like indicated above. For every case it is needed to assess the optimal combination of techniques to be used.

For human specimens,  complementary methods to confirm the findings would yield confidence in data quality.  Availability of pure analytical standards of MNPs, internal standards (when mass spec methods are used), proper sample prep and analyte enrichment methods and suitable instrumental methods are ideal combinations.  Mass spec methods can be completed with FTIR or Raman spectroscopy.

This question would be better stated: What would the most ideal combination of methods be to ensure confidence in both morphology and chemical ID?
Raman or FTIR and PY-GC/MS/MS
Py-GC/TOF MS for nontargeted analysis
PY-GC/MS/MS for targeted analysis

Multiple complimentary methods would provide more confidence for chemical ID (spectroscopy followed by py-GCMS). Confidence in particle size could be increased by measuring the sample with multiple methods as well (FlowCam and then spectroscopic mapping).

For chemical ID, it should be confirmed by at least two, preferably three methods, such as vibrational spectroscopy, pyrolysis-GC/MS, and thermal analysis like DSC. For size, it should be combined with visual observation through microscopy and more counting based light scattering/diffraction method.  

Physical characterization and chemical characterization.