Optimum particle size

I don't have the luxury to run a series of experiments. If there is a rule of thumb or SWAG to go by, please let me know. I want to cast a
billet of aluminum containing hard particles. I will roll the billet down to a foil 1 mil (25 microns) thick so obviously the particles can't be bigger than that. The particles need to be as large as practicable for the effect I want (they are not for dispersion hardening but the idea is the same) but small enough for easy rolling and for a fairly uniform dispersion in the alloy. If I can't roll good foils, it is worthless. Assume the particles are spherical at about 20-30 volume % of the alloy.
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