How can I estimate the market size for a platform business that combines product information with consumer behavior information?

The platform uses a barcode reading mobile app to combine product information with consumer behavior (with full information). We are struggling to understand what is the size of our potential market (BeNeLux + EU), given that we want to sell market reports to brands, on the real drivers behind consumer choice - FMCG and retail in particular.
Any suggestion helps, and I would be happy to talk further!
Thanks!