public double Size {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.DiskShaded;
var pointSize = new CIMPointCloudFixedSizeAlgorithm()
{
  UseRealWorldSymbolSizes = false,
  Size = 8
};
def.Renderer.PointSizeAlgorithm = pointSize;
//Commit changes back to the CIM
pcsl.SetDefinition(def);