How can I use ffmpeg from youtube-dl?

[Soroid]

I want to use ffmpeg's
-acodec copy
within the same youtube function
to convert youtube videos from mp4 to m4a
This would be helpful so i only need to input one command to download a video and extract its audio

I don't want to use youtube-dl's -X because
A) the file size is noticeably smaller so I don't know if there's something 'less' about youtube-dl's audio quality
B) youtube-dl doesn't keep the thumbnail pic, but ffmpeg does.

EDIT: Nvm i found out that ffmpeg actually keeps the video when it converts thus the file size...
How can i keep the image thumbnail after doing -x on youtube-dl then?

[phihag]

You can just run youtube-dl with the -x or --audio-format m4a options.