The bigger the boat and higher wind the more rope you need to let out. I have let out 50-60' of rope to hold me in 15' of water with a 2-3' footers. Also make sure you tie off on the bow not the stern.

I would also use a fluke anchor not one of those round river anchors. Those don't hold worth crap unless you have a pile of rocks to anchor too. It is also up to you if you want to use a chain from the anchor to the rope. Some do to get the anchor to set but if you just let out enough rope it will set and you don't need a chain.