Use something like this script to monitor the size and free space of the files.

Setup a job that runs everyday/week insert into a database to monitor/track over time, send you an email or both.

SELECT

[name]

, [physical_name]

, CAST(( / 128.0) AS decimal(18,2)) AS FileSizeInMB

, CAST((fileproperty([name],'SpaceUsed')) / 128.0 AS decimal(18,2)) AS SpaceUsedInMB

, CAST((( - fileproperty([name],'SpaceUsed')) / 128.0) AS decimal(18,2)) AS FreeSpaceInMB

FROM sys.database_files