Although this topic is pretty much dead, I will post something about it.
Vice City is bigger than Liberty City if you think about it Vice City has 3 Islands (West Island, Middle Island (Starfish, Prawn & Leaf Links & East Island). The bridges are indeed long (One bridge runs from east to West) some take minutes to cross whereas in Liberty City it takes a short amount of time to get from North Portland to South Portland. It takes just slightly more time to get to North Vice Point to South Ocean Beach. Therefore Vice City is longer Width-Wise and Higher Length-Wise.
This is what I believe.