How big is the map?
Rockstar have said the map will be big. To get a sense of exactly how big here is a map mockup overlaid (to scale) onto a real world map. Use the search box to find a real world location you are familiar with.
Accuracy

GTA V

Based on a land area of 51.1264km2, the combined size of GTA IV, GTA SA and Red Dead Redemption (source [1] [2]). Since the official map hasn't been released yet the shape is based on GTA5OClock's mapping project.

GTA IV

Based on a land area of 8.3512km2 (source: [1])

GTA SA

Based on a land area of 28.6889km2 (source: [1])