I have seen many, ranging from about 10 cms (4") to 1 metre in diameter, when I was a kid: but only ever remember one that was a water stone. My dad had a portable, clamp on the bench type: it had a rest for the chisel, axe or knife etc. Playing with it I discovered that it was not as easy to turn with one hand while controlling the bladed implement with the other. I hardly ever see or hear about any, these days but did see a wooden framed (extremely heavy) large one at a second hand dealer a year ot so back : It had a good quality (hardly used) stone about 15 cm wide and about 90 cm diameter. I almost bought it but thought about storage & usage so was a no go for me.