Don't buy an anchor for the mild conditions you want to use it in, but rather for the worst conditions that you may have to use it in. This means buying an anchor first that is relatively large and unwieldy as your boat's primary anchor that will hold in winds in the 40 knot range. Then, and only then, buy a second anchor that is more convenient to use in pleasant conditions. There are several theories on how the force on an anchor system varies with the size of the boat. Some experts relate it to boat length, while others use beam as the critical dimension.
Some people suggest that force goes up by the square of the boat's length.
This means that anchor size must increase much faster than the length of the boat would suggest. Other people frequently use 30 knots as a guideline, or do not state the conditions for which their recommedations apply. Unfortunately, there is no Ohm's Law, or Rules of the Road, or other definitive rule for sizing an anchor for your boat. However we would like to point out Anchors are safety gear. Don't scrimp and buy cheap, poorly engineered and constructed junk.