Rarely anything about human condition is binary. Narcissism, like all other mental illnesses, is a spectrum, indeed.
> At what point do you think people "cross over" to meet a threshold of being a "narcissist"?
I am not a psychiatrist so this is just my opinion, but as a rough approximation, a person is a "narcissist" if their insistence on having superior status to others starts impacting both them and people around them - a person who constantly has to state how superior they are to others, as if they're trying to remind others of how good they are and not letting them keep it out of their mind, is who I would suspect to be a narcissist. Also a person who can't handle any criticism without immediately pointing out how good they are despite the criticism, or getting angry and putting down the person who gave them that criticism. I could come up with more examples, but I hope you see the pattern.
The main marker of narcissism is the inability to not be praised. It actually stems from deep insecurity - deep down, narcissists actually believe that they're inferior in some fundamental way, so the narcissism is actually a self-defense mechanism that helps them not be miserable. It's basically a compulsion, and compulsion cause irrational behavior. That's why the small hands incident made me think of that.
There isn't a threshold, because as you said, it's a spectrum, and defining a threshold would make it binary. But Trump, from my perspective, is deep in the narcissistic territory, far above any regular people I know in my life. As a performer, it helps him a lot - narcissists are usually very charismatic and sure of themselves, making them excellent performers. But that doesn't make him less of a narcissist.