I couldn't find a way to change the document width and height from another diagram.

I am using another dialog, when a user calls it, the user will enter the document width and height then that dialog will update form1. You can assume the dialog is form2.

In form2, I do something like that

public NDrawingDocument DocumentSetting {get; set;}

then I have OnOK

DocumentSetting.Width = Convert.ToSingle(spinEdit1.Value);
DocumentSetting.Height = Convert.ToSingle(spinEdit2.Value);

In form1, I call form2 like

new Form2(this.nDrawingDocument1);

Does not work