Just to add onto Someb0dys comment, we make worldguards as big as we think is necessary (we don't want to worldguard an entire biome for a player because he wants to build an entire city, to months later find out he only made one house). This also means that if you are a player known to make big projects, then you're more likely to get a big worldguard immediately.

Like Someb0dy already said, you can always ask for an expansion of your worldguard. As long as you still use the entire area that you have been given, you can just keep getting your worldguard expanded.