If the database already exists, you can simply right click on the database and select properties. This will show you the space used, and the free space. Subtract the two and you’ll get the space used.
If it’s for a new database, there is an <a href=”http://searchsqlserver.techtarget.com/tip/0,289483,sid87_gci1289528,00.html”>article</a> over on SearchSQLServer.com which shows how to calculate the space needed. It’s based on SQL Server, but the process is the same for all platforms.