There's a difference between market size and TAM (total addressable market) that's worth thinking through.
Let's say you provide SaaS software to the Field Servicing industry (companies with fleets of workers that go around and repair stuff). This represents about $20B in annual revenue in Australia (a number sourced from an industry lobby / advocacy group).
You want this number to be very big. You'll see why as we go a few more layers down: the numbers get smaller quickly, and you don't want to try and build a VC-size company in a small market.
So if all the companies in Field Servicing generate $20B in revenue, what might they spend on our SaaS platform? It's unlikely they'll spend more than eg 2% of their total revenue on their software platform (you can play with different numbers here but it's unusual to see it much higher in the SaaS space).
That makes our TAM: $400M. Not bad right? Well ...
Companies that really nail the product and execution can expect to take out 5-10% of their addressable market (there's not many stories of companies that take our bigger majorities, though it's certainly possible).
So if we say we expect to be able to capture 10% of the market then our near-best-case potential revenue is \~$40M. This is fine, but it's not great: you'll need to look at international growth as part of doing a major funding round.
This is a handy number to sanity check against when you're modelling out your growth. If you model blows way past this, then you need to go back and check your assumptions.