GTA 6 takes place in the fictional state of Leonida — Rockstar's modern Florida. Vice City is the urban anchor, the Leonida Keys are the coastal chain, and at least one secondary city plus a wide hinterland round out the map. Here's the canonical reference, sourced and updated.
GTA 5's San Andreas is roughly 49 square miles, with about 20% urban density. GTA 6's Leonida is probably comparable in total area but materially denser — and traversed differently. Here's the comparison.