I like an entrepreneur who tries to define the market as tightly as possible. This is mainly driven by a loosely defined industry. “I’m addressing the $300 billion US energy market. We want to have people 10% on energy. Therefore our market size is $30 billion” or “I’m addressing the $2.3 trillion US services market. It is a huge market.” That is a sure way to lose the interest of a potential investor. Define your market sizing as tightly as you can. Nobody will fault you for estimates, just make sure they are reasonable.