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.
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.