I recently came across this problem when I DM'd a ready-to-use oneshot - its main battle took place on the town square, and the map supplied with the rest of the campaign guide would have required me to use 4 flipchart-sheets (which are already at ~1m² each).
I ended up ignoring that map and just drew a custom map on one flipchart-sheet, but that town square was only around 20 squares across - and 100ft. / 30m is not that huge for a town square.
Anyway, since I'll be starting a real campaign next week (as the DM), I figured I might run across this problem again, eventually. Of course, I could just avoid planning battles in big, open spaces, but I don't want to limit myself or the story plot because of issues like this.
Just using an empty flipchart-sheet or no map at all is of course also always viable, if the space really is just wide open, but that might not always be the case. Also, I love drawing maps, so if there's a solution that allows me to use maps without having to use under-proportionally small town squares or similar, that would be great, so - thanks for any suggestions in advance!