So calculates Ryan Cooper:
What I did was take each state’s population compared with its electoral votes, find the population per elector, and rank them by greatest over-representation. Then, I added up electoral votes until I got past 270—as it turns out, 39 states makes a pretty close 274. (I imagine it would be possible to shave a few tenths off my percentage by playing with the totals, but this has to be pretty close.) I assumed the winner got 50% + 1 votes in each of the winning states and no votes in the remaining states, giving the totals on the right.
But even this is an overestimate since differential turnout is possible. You could win California with seven votes if only 13 people bothered to show up on election day.