I don't know if it is the same as for a water in oil emulsion (Heavy fuel oils for large diesel engines) but here i have been told that when using a fuel mill or emulsifier water droplets below 50micron will form a stable emulsion. larger droplets are removed in the centrifuge.
S.I.T. may be able to help, they are a German company supplying fuel water emulsifying equipment.
Emulsions are a hot topic with large diesel engines so visiting the web site of CIMAC you may find some articles there.