Is there a property to control the Size (Width/Height) of FlowChart elements that are derived out of GraphLinksModelNodeData?
That depends on the DataTemplate that you use.
If you want to directly control the Width and/or Height of any elements from the node data, then you certainly can add those properties and data-bind them, as several samples do.
But you might want to set the MaxWidth and/or MaxHeight of the elements in your DataTemplate.