Depends on what you do with the boat, but for diving from my Achilles 14' (unattended boat), I used to use a 5lb Danforth knock-off. On my 18' RIB, I use a 8lb Danforth. Those do take a up much more space than a claw type. I would thing that a 4 lb (or 2 Kg?) claw would do fine.
Just as important is using a decent amount of chain to keep the pull on the anchor horizontal.
jky
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