USE: To display database file size information at the server level.

SELECT @@SERVERNAME [ServerName],
       MF.database_id,
       DB.[name] [DBName],
       SUM(CAST((MF.[size] * 8.0) / 1024.0 AS numeric(19,2))) [Total Space Usage (MB)]
  FROM sys.master_files MF
 INNER JOIN sys.databases DB ON MF.database_id = DB.database_id
 GROUP BY MF.database_id, DB.[name]
 ORDER BY MF.database_id
 GO