Opinions will differ, and where you use your boat and anchor will play a part, but on my Merry Fisher 605 (used in the Solent), I had a 5kg D-type anchor with 6m of 6mm chain, plus 45m of 10mm rope. It always held tight.
I think it was similar to this: D Type Anchor - HD Galvanised - 6 kg - Marine Scene
If you read the 'information' link, it says suitable for boats up to 25ft/7.5m.
MF 695/2 Knox 9.0 kg, 7m 8mm chain, 100m nylon - def over spec'd but i overnight and sleep well this way
nylon bit long but the capstain bites into it and easier to shorten it as and when worn