If you were to speak passionately for 20 minutes about a growth topic what would it be and to what audience?
Maybe you're teaching someone or ranting on a particular common sales technique, what's your topic and the specific audience you would address?
I would rant and rave to freelancers about choosing a frighteningly narrow niche until the 20 minutes ran out, and then you'd have to drag me off the stage with a vaudeville hook 😁
Nice! What's the right balance? What's too narrow? Someone described Costco as being in a niche - cheap bulk buys, in the grocery store category. So your niche can be a large market. Or a niche could be by an audience - i.e. Indiehackers.
Wow, the Costco example really highlights the challenge. TBH I have no idea how one builds a company like that!
But even if the best answer is "drop hundreds of millions of dollars to build huge stores in major metros across the country" that's probably not an option for any of us.
So for starting out I'd say it's hard to go too narrow. If there are at least a couple hundred potential customers in your target market, go for it. An eventually-Costco biz might start out selling bulk frozen vegetables to locally-owned restaurants in a single city.
The balance is achieved when it's really easy to find those prospects, and quickly become an expert on their problems. "All food shoppers" is a hopelessly diverse group, expensive to reach, hard to reason about. "Restaurant owners in Tuscaloosa" will have more in common, be easier to find, and might all need your help.
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(To be clear, strongly in favor of choosing a frighteningly narrow niche)