You’ll need to add your own Width and Height properties to your node data class. And you can data-bind the Width and Height properties of some element in your Node [Template] to these two data properties.
I’m sorry I don’t have access to any of our development systems at this time, but perhaps some properties besides Diagram.AllowGroup or Part.Groupable are preventing grouping. Does CommandHandler.CanGroup() return true? If not, perhaps the model is not Modifiable.
Also, check Diagram.IsReadOnly and Diagram.AllowInsert.