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Wikipedia's Soap gives sodium stearate as an example of soap, and apparently I've been eating it:

Sodium stearate is the sodium salt of stearic acid. This white solid is the most common soap. It is found in many types of solid deodorants, rubbers, latex paints, and inks. It is also a component of some food additives and food flavorings.

What would be the smallest or simplest molecule that we could reasonably call a soap? Perhaps a functional definition would be that it could perform some of the functions of soap in the same way that soap does.

In Why does bleach feel slippery? and its follow-up Is it known for sure that bases feel slippery because of the production of soap/surfactant? the saponification of other existing molecules is discussed, and I'm not looking for that here.

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It boils down to the definition of soap. Wikipedia defines a soap as the salt of a fatty acid.
IUPAC claims the smallest fatty acid can be considered to have 4 carbons.
Therefore the simplest soap molecule would be a (generally sodium) salt of butyric (butanoic) acid, i.e. sodium butyrate.

Now apart from the chemical definition, a soap must adhere to its function in order to be defined as such. Therefore, before calling something a soap, we would have to know if the substance does in fact act as a surfactant in a oil-water surface (reducing the interfacial tension) - but that will depend on the nature of the oil in question and the purity of the water. Only for a well defined system we can then conclude that such molecular salt is acting as a soap in it.

While I agree that the definition matters I disagree with the definition of soap as "as the salt of a fatty acid". For one SDBS, AOT, SDS, Cocamide DEA (not even a salt) and CTAB are all popular soaps that are not fatty acid salts. I also do not think that soap and surfactant are interchangeable words. Surfactants lower surface tension, soaps create micelles either by them selves or with oils. Essentially all soaps are surfactants but not all surfacants are soaps. For example, ethanol may be considered a surfactant as it does reduce the surface tension of water, but it does not help form micelles in water appreciably and thus should not be considered a soap.

What would be the smallest or simplest molecule that we could reasonably call a soap?

This is somewhat a matter of estimate, but given that propionic acid is not micible with water and acetic acid is, I would assert that sodium (or other alkali metal) propionate is the simplest soap since it has an aliphatic group to interact with an oil phase, yet has carboxylate salt "head" to retain interaction with the water phase. It is possible sodium acetate may do so as well but empirical data is needed. I suppose technically lithium salts would be the smallest since the lithium ion is smaller than the sodium ion, but that is perhaps too technical and may not interact with water as strongly as sodium salts.

As always, there is no sharp border and no minimal molecule. You can make the head piece more charged, which means the molecule becomes more hydrophillic and water soluble and you can make the tail piece shorter, so it will not bind fats very well anymore and fails to form micelles at some point.

Consequently, the question makes no sense, because the issue is a task-specific trade-of and the term soap is too broad. It is not even clear, whether you want a detergent for water or a e.g. fluorinated oil.