I know its a "lame" question, but: lets say, I have a wmv, avi, etc BIG file. I want to convert it to .MP4, with the possible smallest file size, and of course with the same quality with audio/video. How to? What codec should I use? Im using FFmpeg
You can try to encode with libx264 crf 18. It is well known for producing a (arguably) "visually lossless" result.
ffmpeg -i input.avi -vcodec libx264 -x264-params crf=18 -preset veryslow -acodec copy out.mp4
Notice that I used a veryslow preset which will give you the smallest file possible. Also, I decided to copy the audio track instead of reencoding it, mainly because I assume it's already AAC/MP3 stereo in your sample.
If the output is too big for you, try with a higher crf value (up to 23) but the quality will suffer.
Getting the smallest video with the least amount of quality lost is more of a challenge than anything else. There are guides scattered among the Web that will recommend you settings to use on FFmpeg for things like game captures, streams, DVD rips, etc.
If you want lossless lossless, try this:
ffmpeg -i input.mp4 -c:v libx264 -crf 0 output.mp4
The important part is the
-crf 0 part which sets it to lossless. However, the output file will generally be very large. You could change it to
-crf 18 which would give you something that is acceptable in quality and free from most artifacts.