Gets the size of all the databases on a given sql server instance. (Credits: http://goo.gl/4aw6Al)

all_databases_size.sql

SELECT      @@SERVERNAME AS SqlServerInstance,
            db.name AS DatabaseName,
            SUM(CASE WHEN af.groupid = 0 THEN 0 ELSE 8192.0E * af.size / 1048576.0E END) AS DatabaseSize,
            SUM(CASE WHEN af.groupid = 0 THEN 8192.0E * af.size / 1048576.0E ELSE 0 END) AS LogSize,
            SUM(8192.0E * af.size / 1048576.0E) AS TotalSize
FROM        master..sysdatabases AS db
INNER JOIN master..sysaltfiles AS af ON af.[dbid] = db.[dbid]
WHERE       db.name NOT IN('distribution', 'Resource', 'master', 'tempdb', 'model', 'msdb')   

GROUP BY    db.name