Hi Rohit,
This should be possible with FFMPEG. You may need to add a specific step to the Update Asset Workflow for this piece. You can practice the syntax via the command line interface for FFMPEG until it give you the desired output and then put into the AEM workflow step to try to isolate which system is giving you problems. I've also found it helpful to create a one-step workflow when trying to find the syntax--you can run it against your payload a lot faster than the full Update Asset workflow.
https://trac.ffmpeg.org/wiki/Scaling - this is their documentation and it covers scaling.
This is also possible with Dynamic Media, but I presume since you mentioned FFMPEG this isn't licensed for you.
Hope this helps,
Beau