I thought the Rocna had you at a 10kg. I'm guessing you have a decent length of chain so would think a 10 would be good. If you want a little more go the Supreme 25, if you want to
head into the big area go a Rocna 15 or very serious with a Supreme 35. Each
anchor is a little bigger in physical area each step so a handy incremental performance increase depending on your desires.
But don't forget about the physical size when not in use.