One method to size a market is to use existing reports. There are roughly three different categories of firms that analyze markets that can help to size a market. 

Market Research Firms

• Written by analysts and researchers to cover many different industries
• Written for large firms and universities who need broad information about a market
• Size information might be too broad to be helpful and potentially inaccurate

Investment Analyst Firms

• Written by analysts who specialize in an industry and research everything including rumors
• These analysts make projections to help clients know whether to buy/sell/hold
• There's potential bias to overinflate market size since these reports are sold to the investment community

Market Analyst Firms

• Written by analysts who have worked in these industries
• Tend to be considered "household names" in the business world
• The connections these analysts have mean that the market sizes they develop are considered the "gold standard" and reliable. 
  • The downside is that they cost a lot and are harder for universities to get

Information taken from "Chapter 23 Market Sizing Activity" by Meryl Brodsky in Teaching Business Information Literacy under a CC-BY-SA license. 

Snipes, G., Karo, M., & Faulkner, A. E. (Eds.). (2022). Teaching business information literacy. Association of College & Research Libraries.