There is a field of economics that thinks about this set of issues.
Apart from the obvious criteria like language, culture and political unity, a suitable case for a single currency is thought to be stronger when:   the forces (“shocks”) that affect a group of regions or countries are fairly similar and the way in which the regional economies respond is similar;    there is a lot of trade between the regions (as there usually is within a single country);    factors of production (labour and capital) flow fairly easily between the regions in response to differences in conditions; and/or    when other means for responding to differences in experience (particularly fiscal transfers) are available.