Measurement for the downstream use of a commodity I am looking to find a measurement that would tell me the size of the market of the downstream use of some particular commodity. For example, salt or wheat would be used by many companies downstream. Mangoes on the other hand will be used by less firms downstream. Therefore, there is a larger downstream market for salt and wheat. Are there existing measures that have been used to measure the size of downstream market of some particular commodity? If not, do you have any suggestions on how to build one?
If you are looking for certain commodity data from the US.  The USDA Economic Research Service has data on market size of certain commodities.  But I am not sure about firm level data.  

https://www.ers.usda.gov/data-products/commodity-costs-and-returns/

There measures include cost and return estimates are reported for the United States and major production regions for corn, soybeans, wheat, cotton, grain sorghum, rice, peanuts, oats, barley, milk, hogs, and cow-calf. The series of commodity cost and return estimates for the U.S. and regions is divided into two categories: Recent and Historical estimates. 

I am not sure if this is exactly what you are looking for but this is a good start.  One can also call their office for additional information.