I believe it has a very extensive map. If you are thinking about buying the game though, have you checked out GTA: Chinatown Wars?
its alot more fun either way, since it's in 3d. the driving is alot more entertaining. the fun part only pertaining to exploring of course.
The map is very small.I first thought it is big, but then.... Well...If you know the map of gta chinatown wars or gta4 there is a part of an island at the upper right . It is around half as big as 1 of the 2 towns in Gta Chinatown Wars.This should be the size of the gangstar map.
It has a big map, but if you're looking for a map, get GTA for sure. It's probably 4 times bigger and TONS more interactive.
The map in gta is way bigger and the game play is much better as well. Gangsta was boring and just felt so empty in comparison, and although it had a TPS view I didn't feel the graphics were better, they were worse. The draw distance was average and I never even got to finishing the game as I just got over it, and I'm a huge GTA style game fan. It's not a bad game though, just don't expect it to be anywhere near as good as the real GTA.
Gang$tar is still worth getting even if if you've played Chinatown Wars, much smaller map than GTA (and gets a bit claustraphobic if you're into exploration) but if you focus on the missions you'll hardly notice and it's a damn good game.
Gangstar's map is pretty and in 3d, but it is not very big. There's also not much to do. Not many people (both in quantity and in variety) walking on the streets and they don't really do anything, just walk mindlessly back and forth. No day and night like in GTA, empty streets that look nice but you can't really interact with any part of it. The draw distance is also notoriously bad in gangstar. GTA's map is both much bigger and feels much more "alive." The people have different animations, you can run over trash cans, cause havoc, etc etc. I played through all of gangstar back before GTA came out though and it was a fun game in its own right, but GTA is much better IMO.
You can explore the whole city from the start. It isn't as good as GTA: CTW, but I'd still highly recommend it and it's worth buying if you have finished with GTA.