http://www.newsweek.com/red-dead-2-rdr2-map-size-vs-gta-5-v-1190240

Rockstar Games has been a little cagey on the exact size of Red Dead Redemption 2’s map, preferring people discover it for themselves, but it’s almost certainly the largest they’ve ever created.

Red Dead Redemption 2 even includes much of the map from the first Red Dead Redemption—essentially everything north of the San Luis River—which allows us to draw some very rough comparisons from one map to another.