How to convert to FFMPG for a specific size?

Question

Video processing

Greetings, there is a large array of short video files in webm format. They need to be converted to mp4. The problem is that when converting, their size increases, and I cannot exceed 50 mb. You cannot compress video with significant loss of quality.
How to influence ffmpg so that it calculates before converting whether the final video will be up to 50 mb or not. Or is it possible to re-encode without increasing the size?

Answer 1

Use two-pass encoding with precomputed bitrate
https://trac.ffmpeg.org/wiki/Encode/H.264#twopass