How do you estimate market size for a new B2B SaaS tool?

For cybersecurity SaaS, I'd start with Gartner and IDC reports for top-down TAM figures, then cross-check with LinkedIn Sales Navigator to count actual companies fitting your ICP for a bottom-up SAM. Crunchbase and G2 category data are also great for sanity-checking competitor revenue estimates. The top-down vs. bottom-up triangulation is usually what clients find most convincing in a pitch.