pacdep is a small tool that will allow you to quickly see how much space is being used/required by a package (and its dependencies).
By default, it will give the installed size of the package, its exclusive dependencies, and its shared dependencies; Optional dependencies can be included as well.
pacdep searches the local database first, and if not found searches all sync databases. Package names are prefixed by the repository name they were found in when they're not from the local database.
You can have dependencies of a group listed, in which case the packages name and installed size will be shown. You can also show the "dependency path" for each pakcage, using --show-path
After the package's installed size, in parenthesis will be the installed size of the package and its exclusive (and optional, if --show-optional was used) dependencies.
If the package is from local database (i.e. is installed), only dependencies from local databases are taken into account If it is from a sync database (i.e. is not installed), only dependencies from sync databases are taken into account.
In other words, this size represents either the size the package and its dependencies are using on the system (size that could potentially be freed if removing the package and its dependencies), or the size needed to install them.
Some useful links if you're looking for more info :
Plus, pacdep comes with a man page.