There appear to be several halachot that are based on "a majority of Israel". For example:
הרואה מקום שנעשו בו נסים לישראל אומר ברוך שעשה נסים לאבותינו במקום הזה
IF ONE SEES A PLACE WHERE MIRACLES HAVE BEEN WROUGHT FOR ISRAEL, HE SHOULD SAY, BLESSED BE HE WHO WROUGHT MIRACLES FOR OUR ANCESTORS IN THIS PLACE (Soncino translation).
The Talmud there qualifies this:
אמרי אניסא דרבים כולי עלמא מיחייבי לברוכי אניסא דיחיד איהו חייב לברוכי
The answer [is that] for a miracle done to a large body it is the duty of everyone to say a blessing, for a miracle done to an individual he alone is required to say a blessing. (Soncino translation)
Rabbeinu Yonah there defines a ניסא דרבים as a miracle that was done for all or most of Israel:
כלומר שנעשה לכל ישראל או לרובן
So let's imagine that a major miracle occurred. How would we know if it meets the threshold of "most of Israel". Are we expected to travel the entire world to figure out the Jewish population and then calculate whether the miracle occurred to > 50% of that number? Are we just supposed to assume that it was not "most of Israel" unless it is glaringly obvious (e.g. millions of Jews were together such that it is virtually impossible that this is not the majority)? Or is perhaps the threshold of "most of Israel" not exactly precise and it does not actually require a technical majority? Or perhaps it only refers to certain Jews (e.g. those living in the Land of Israel)?
Are there any sources that discuss the parameters of "most of Israel", or how we would go about determining if the threshold was reached?
Note that an answer need not address the specific case above. There are other situations where this could be relevant as well. For example, the Sefer Hachinuch writes by a bunch of mitzvot that they are applicable בזמן שרוב ישראל על אדמתן – when the majority of Israel is in its land (e.g. Mitzvah # 95).