To calculate the "size", "used size in" and "free space in" a Microsoft SQL Server 2008 mdf or ldf file use:

use master

go
select
    name,
    cast((size/128.0) as int) as TotalSpaceInMB,
    cast((cast(fileproperty(name, 'SpaceUsed') as int)/128.0) as int) as UsedSpaceInMB,
    cast((size/128.0 - cast(fileproperty(name, 'SpaceUsed') AS int)/128.0) as int) as FreeSpaceInMB
from
    sys.database_files

Result

For more information on fileproperty, see: http://msdn.microsoft.com/en-us/library/ms188401.aspx