There should be a measure of an economy that is calculated by adding absolutely every active economic activity that occurs in that economy. Things like the sale of goods and services, the sale of intermediate goods between businesses, garage sale activity, and absolutely all other active economic activities between any and all entities would all be added together. The resulting figure would be called the ‘economic sum’ of that economy.
Perhaps illegal activities like drug sales and prostitution could also be calculated and placed under a different subcategory (such as under the ‘black market’), but still added to the ‘economic sum’ because they are also active economic activities. Statistical sampling could probably be used to accurately determine the size of such unregulated economies.