One of the best way to calculate size of all database using SQL server management studio query editor.
SELECT sys.databases.name, CONVERT(VARCHAR,SUM(size)*8/1024)+' MB' AS [Total disk space] FROM sys.databases JOIN sys.master_files ON sys.databases.database_id=sys.master_files.database_id GROUP BY sys.databases.name ORDER BY sys.databases.name --or exec sp_spaceused
You may look for Calculate all tables Size in database in SQL server Syntax.