resize a video to 1080p with ffmpeg and the scale and pad filters
ffmpeg
-i infile.mp4
-vf "scale=-1:1080,pad=1920:ih:(ow-iw)/2"
outfile.mp4
scale and pad a video to 1080 batch process
find -s . -type f -name "*.mp4" -exec sh -c \
'ffmpeg -i "${0}" -vf "scale=-1:1080,pad=1920:ih:(ow-iw)/2" "${0%.*}-1080.mp4"' \
"{}" \;