The article that mentions the magic number 49 has mistaken KM for MILES!
49km = 30 miles.
If RDR is around 6m sq...
GTAV landmass is 21m sq (roughly RDR x 3.5)
The water is 10m sq (roughly RDR x 1.5)
31 miles square converted to km is 49.8. The number 49 is close to accurate - the unit was wrong!