BuzzFeed have caught an exclusive interview with Rockstar North's Art Director, Aaron Garbut. Aaron is the man responsible for how GTA (since GTA III) looks and feels - so he's a pretty important bloke. Really interesting read which includes 5 brand new screenshots which we'll hopefully get higher res versions of later! Some choice quotes are below.
 

In comparison to GTA IV:

We used to call Liberty City a living, breathing world, but this is way beyond anything we've done before.

 
On the size and detail of the game:

It’s levels of magnitude above Grand Theft Auto IV in terms of what we’ve done on the environment side. I know there are bigger games out there geographically, but I don’t think there are in terms of content. I want to stress that not only is this world huge but it’s absolutely handcrafted. Every little bit of this world has had a large number of extremely talented artists pore over it. There’s always something to discover, something weird or interesting to see or interact with. It’s absolutely not a massive, empty world. We’ve considered the placement of every tree. We’ve simply not copied buildings around the map or procedurally generated the terrain to pad it out. It’s all handcrafted, all unique, and we’ve gone over it all again and again and again to make sure there’s enough layering of detail that I don’t think many people will ever see everything we’ve put into the world. That in itself, though, means that most people will have different experiences.

 
Read the full interview.