Elaine, Consider side scatter as a selection parameter . . . Regarding their size . . . I've run 50 nm particles, but had to rely on fluorescence to detect these initially. I found 100 nm particles with -out fluorescence, but it's not easy.
Uniformity is key . . .
This makes sorting even more difficult, but I'd worry about that later. Going back to the reference of bead development . You could then monitor the accumulation be fluorescence increase. These are tiny, little 100 nanometer particles. The free glass spheres nucleate and grow under the reaction conditions for the growth of the core-shell particle.