This script provides File Size, Space Used and Free Space avaialable for each file like Main file, Log file and each partition file in a database.
Below is the screenshot which I see in a database having 12 partitions, 1 each of every Month.
Find Size of files used in Database
SELECT
Name
, Filename
, convert(decimal(12,2),round(size/128.000,2)) as FileSizeMB
, convert(decimal(12,2),round(fileproperty(name,'SpaceUsed')/128.000,2)) as SpaceUsedMB
, convert(decimal(12,2),round((size-fileproperty(name,'SpaceUsed'))/128.000,2)) as FreeSpaceMB
FROM dbo.sysfiles