If you're new to anchoring, the answer is probably "a lot smaller than you think". Boat anchors have come a long way since the time of the Popeye like "Navy Anchor". Today, a modern anchor design weighing just 10 lbs or so can securely anchor a boat 20' or bigger.
We list in the table below the four most popular boat anchor types, the most popular sizes in that group and the corresponding size of boat that size/type of anchor is good for. These ratings are based on an average displacement boat anchoring in average conditions. If you're unsure if your boat's displacement and/or your anchoring conditions are average, the chances are it is average.