Has anyone found a second source for the map size figures
The three times RDR2 and twice Los Santos numbers both come from the same interview with the same person.
That is one source. It is a good source, it is Rockstar's own development lead, and it is still one source quoted in about four hundred threads as though it were a published specification.
Asking whether anyone has found the same claim made independently anywhere else, because if not we should all be a little more careful about how we phrase it.