To edit the sizing of Cluster Points, head to the Styles tab in the Layer Editor. Navigate to the Points section of Global Styles, and then turn on Cluster Points at the bottom of the section. Once it is on, if you click the drop-down arrow to the right of it, you will see an option for Proportional Sizing. Turning that on allows you to add stops based on the number of points and a size that it refers to, that way you can create ranges of points that are referenced to a certain size of point on the map, allowing for greater data visualization.