path.pointRadius()

path.pointRadius([radius])

If radius is specified, sets the radius used to display Point and MultiPoint features to the specified number. If radius is not specified, returns the current radius accessor, which defaults to 4.5. While the radius is commonly specified as a number constant, it may also be specified as a function which is computed per feature, being passed the any arguments passed to the path generator. For example, if your GeoJSON data has additional properties, you might access those properties inside the radius function to vary the point size; alternatively, you could d3.symbol and a projection for greater flexibility.