Querying True Physical Database File Sizes
Here you go.  You can use this to get file sizes by file and filegroup.  It includes the system file size and how much space it's taking on disk, as well as the SQL file size, SQL space used, and by extension SQL free space as well.  It includes the full path of the file being evaluated.

SELECT fg.data_space_id AS FGID,
   (f.file_id) AS File_Id,
   -- As provided by OP, size on disk in bytes.
   CAST(f.size AS FLOAT) * 8.00 * 1024 AS Size_On_Disk_Bytes,
   ROUND((CAST(f.size AS FLOAT) * 8.00/1024)/1024,3) AS Actual_File_Size,
   ROUND(CAST((f.size) AS FLOAT)/128,2) AS Reserved_MB,
   ROUND(CAST((FILEPROPERTY(f.name,'SpaceUsed')) AS FLOAT)/128,2) AS Used_MB,
   ROUND((CAST((f.size) AS FLOAT)/128)-(CAST((FILEPROPERTY(f.name,'SpaceUsed'))AS FLOAT)/128),2) AS Free_MB,
   f.name,
   f.physical_name
FROM sys.database_files f 
    LEFT JOIN sys.filegroups fg
    ON f.data_space_id = fg.data_space_id


For more information on the difference between 'Size_On_Disk_Bytes' and 'Actual_File_Size', go here: https://superuser.com/questions/66825/what-is-the-difference-between-size-and-size-on-disk

Thanks