What is the GTA 6 map size?

Rockstar has not published an official square-mile figure or a complete measured map. The official site describes Vice City and the wider state of Leonida, and its location pages introduce places within that setting; those announcements do not establish a total area or a reliable comparison with earlier games.

Until Rockstar publishes a measurement, any exact area or comparison should be treated as an estimate rather than a confirmed fact. See Rockstar's GTA VI locations and Trailer 2 announcement.