Even if Miami isn't such a big city, VC can be done bigger. And it needs to be bigger, they can't make a map that's smaller than V's.
The city can be around the same size, but the countryside should probably be smaller. Florida doesn't have mountains or deserts like California. There are just some wooded areas and swamp. If they made the map the same size as V's, it would probably have large areas that feel useless. Even with the variety of areas they had to use with a southern California setting, a lot of areas in GTA V's countryside could be removed without being missed.
They need to focus on content and filling the map with things to do rather than making large useless areas.