Considering the modeling the system can do I'd expect a block or two draw distance Liberty City but scaled down some, fewer people on the street (no mobs), and some cars. The system can do things like PSX and N64 could so it's possible to create a pretty large place with a ton of moving lower polygon objects...draw distance vs frame rate will be the ultimate issue.
GTA Advance was fantastic in that it was GTA3's City copied into a top down view whereas the entire building/terrain structure were all 3D textured (and some animated) polgyons and all moving objects (cars, bullets, fire, people, random blood spatter, etc...all sprites. Given the difference in what the GBA and DS can do they're going to have to go well beyond the quality that title has or people will be pissed.