i have just added a 8gig movie to my hard drive but plex will not find it when i do a scan on the movies file it is the largest file i have most of the other s are less than 4 gig
What is the file extension....avi, mkv, other?
First of all, make sure your file is named correctly and it follows the Plex naming guidelines. See the 'File Naming in Plex' link in my signature for more details.
For Movies, they should be named like so: Movie Title (2018).ext
(2018) is the year of release for that Movie.
The file size has no bearing on anything at all. File size at best, only indicates the level of compression the file has.
DSM has a habit of losing permissions. Go to DSM Control Panel > Shared folders and make sure your media folders have R/W permissions for both admin and Plex.
Also, see Q18 on the Synology FAQ's page and RESET your file permissions, link in my signature.