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

The mistake you have made is that in your form's constructor you assign the property to the passed parameter. It should be exactly the opposite ! Assign the parameter to the property, i.e.: DocumentSetting = selectedDocument

Best Regards,
Nevron Support Team