@Zink, All other things being equal, for the same weight of pigment, as particle size decreases, the opacity/coverage of TiO2 or ZnO increases. This holds true until you get down to particle sizes that are too small to reflect visible light frequencies, which is how transparent dispersions for sunscreens are obtained.
That being said, there are two levels of particle size reduction. The first level is de-agglomeration, which breaks up clumps of pigment until you get down to the primary particle size. This level is relatively easy.
The second level is reducing the primary particle size. This is much more difficult and energy intensive, and may not even be possible with any standard cosmetic processing equipment short of a jet mill.
Thanks @Bobzchemist had a feeling you’d chime in on this one (and happy chemistscorner ranks high on google, so many others with the same question are likely to find this thread).
In lieu of a Jetpulverizer or proprietary shaped particles, these are the options I’ve found readily available in stores, so in this case makingcosmetics or possibly soap goods could be good choices with relatively low particle sizes?
barmbleberry: doesn’t list particle size bulkapothecary: doesn’t list particle size essentialwholesale: 263 - 1233 nm