sp_helpfile & size

You could try a before that:

DBCC UPDATEUSAGE
(   { database_name | database_id | 0 }
    [ , { table_name | table_id | view_name | view_id }
    [ , { index_name | index_id } ] ]
) [ WITH [ NO_INFOMSGS ] [ , ] [ COUNT_ROWS ] ]


http://msdn.microsoft.com/en-us/library/ms188414.aspx

good idea- can i run this on a busy system in production, or should wait till maintenance window?

Maintenance window is best.  See best practices in BOL:


Best Practices
Always run DBCC UPDATEUSAGE after upgrading a database from SQL Server 2000. The page and row counts are corrected and are maintained thereafter.


Do not run DBCC UPDATEUSAGE routinely for databases created in SQL Server 2005 or higher or on upgraded databases that have been corrected once by using DBCC UPDATEUSAGE. Because DBCC UPDATEUSAGE can take some time to run on large tables or databases, it should not be used only unless you suspect incorrect values are being returned by sp_spaceused.


Consider running DBCC UPDATEUSAGE routinely (for example, weekly) only if the database undergoes frequent Data Definition Language (DDL) modifications, such as CREATE, ALTER, or DROP statements.

thanks very much.