How do I reduce the speed of a voice mp3 file with sox to 75%?

Try this (with input.wav as your input file)

sox input.wav output.wav speed 1.33

Taken from this, its example 13.

Regarding pitch correction, if the pitch should stay the same, use this command instead.

sox input.wav output.wav tempo 1.33

Regarding filesize. It depends on the audio format you want to use. If it is compressed, select settings for higher compression. If not, you may want to convert it to mono, reduce the sample rate or bits.

I just tried slowing down a typical CD-quality audio file with sox and ffmpeg for a side-by-side comparison of changing tempo without changing pitch.

It appears that sox introduced some stutter that ffmpeg did not. Would recommend ffmpeg, for slowing at least; here at half-tempo:

ffmpeg -i file.mp3 -filter:a "atempo=0.5" file_0.5.mp3