When it comes to kayak anchors, the rule of thumb is that they should be no more than 1/10th the weight of your kayak. So, for example, if you have a 10-pound kayak, your anchor should weigh no more than 1 pound.
Of course, there are a few factors that can affect how heavy your anchor needs to be. One is the type of bottom you'll be anchoring in. A sandy bottom will require a lighter anchor than a muddy one. Another factor is the wind conditions. In high winds, you'll need a heavier anchor to keep your kayak from being blown away.
Ultimately, it's up to you to decide how heavy your kayak anchor should be based on the conditions you'll be paddling in and your personal preferences. But following the 1/10th rule is a good place to start.