ffmpeg -i input.file -map 0:a:0 -b:a 96k output.mp3
...will convert any file with audio into a Constant Bit Rate MP3 @ 96 kbit/s. Music files normally store cover images as a video stream, which will be stripped by this command; M4A files do this differently, but ffmpeg is currently not able to access that data, so it will be stripped whatever you do. This will also select the first audio stream, if there are multiple audio streams.
CBR mode should be faster than VBR, and using a low bit rate should be faster than a higher one.
Of course, file size can be easily calculated from the bit rate. A one-minute CBR MP3 @96 kbit/s will have a file size of
60s*96000bit/s=5760000 bit, /8192=703.125 KB.