I played both 3 & VC & 3 took me longer to explore. In VC you go straight up & down. Make a left to bridge & then you on a really small island & then on 2nd part of map which again is just straight up & down.
3 has more turn,hills,tunnels & overall much better layout. VC was just a flat ass map.
According to a GTA information site, the area of San Andreas is four times larger than the size of Vice City, and five times the size of Liberty City. So therefore, Vice City is larger than Liberty City. But for the GTA IV Liberty City, I don't know if it's about the size of San Andreas, or bigger or smaller. I got a feeling it's the same size when I measured/compared the two maps.