I use the GoPro Hero Session (circa 2016) on a hot shoe mount on my Bessa R4M.
Very small - a bit larger than a pool cue chalk.
It is most used by me to capture a panorama or to capture sounds associated with a film scene.
At day end you can use the mobile app to connect to the GoPro and edit video.
If you are interested in strictly in sound only then get a Xoom recorder and a hot shoe mount.
Here is my GoPro hot shoe
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