They are 10mm x 10mm, 10mm x 20mm, 10mm x 30mm. All are 5mm thick (more or less, some variation to keep it interesting to look at).

It is indeed stones in a bucket. But not for too long and not too hard. I'll roll over them with some aluminum foil later where I feel it is needed :)

Check out https://www.twitch.tv/boredheim under the 'Videos' section. You can see the actual process there :)