They also don't have exactly the same set of commodities as a larger market of the same apparent type - the various Refinery exports can also show up sometimes at Extraction or High-Tech or Industrial settlement types, for example.
The closest you'll get to official documentation of this is this patch note entry
Fixed on foot settlements not having correct max commodities supplied. They will now have approximately 3 for small, 5 for medium and 7 for large
which changed their behaviour to match that of settlements in pre-colonisation systems [1].
As far as unofficial documentation goes I don't think anyone's attempted to make a systematic survey of what exports each settlement type can have, though you could probably get an idea reasonably quickly from the market data in Inara or Spansh.
Also note that changes to system Wealth, Standard of Living and probably also Development Level at least will shuffle
which commodities your settlements produce.
[1] Tourism and Military settlements which are based on economies which don't have 7 distinct exports in the first place have lower limits as you might expect.