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.
GTA V's rural / desert region north of Los Santos — anchor of Trevor Philips's storyline, home to Sandy Shores, Paleto Bay, and the wilderness that makes V's map larger than its predecessors.
GTA V's primary urban core — the modern reimagining of Rockstar's Los Angeles analog city, first seen in 2004's San Andreas, fully rebuilt for V's 2013 launch.
GTA V's iconic mountain in northern Blaine County — and the locus of one of the most enduring community-speculation mysteries in modern gaming, anchored by an in-game mural that hints at hidden content fans have analyzed for over a decade.
GTA V's northern coastal town in Blaine County — Pacific Northwest-coded small community, site of The Paleto Score heist (Bank of Liberty branch target), and V's clearest example of a fully-realized small-town setting outside Sandy Shores.
Trevor Philips's home base in GTA V — a desert-coded, Salton-Sea-analog town in central Blaine County, anchoring V's most explicit satire of inland California / Nevada border culture.