public double MinSize {get; set;}

//Must be called on the MCT
//var pcsl = ...;
var def = pcsl.GetDefinition() as CIMPointCloudLayer;

//Set the point shape and sizing on the renderer
def.Renderer.PointShape = PointCloudShapeType.DiskFlat;//default
var scaleSize = new CIMPointCloudSplatAlgorithm()
{
  MinSize = 8,
  ScaleFactor = 1.0 //100%
};
def.Renderer.PointSizeAlgorithm = scaleSize;
//Commit changes back to the CIM
pcsl.SetDefinition(def);