RTF table border size

I'm doing a simple RTF editor (with RichTextBox ). I'm working on the editing part of tables, the code to insert them is ready:

private void InsertTableInRichtextbox(int rows, int columns,int csize)
    {
        //CreateStringBuilder object
        StringBuilder strTable = new StringBuilder();

        //Beginning of rich text format,don’t alter this line
        strTable.Append(@"{\rtf1 ");

        //Create 5 rows with 4 columns
        for (int i = 0; i < columns; i++)
        {
            //Start the row
            strTable.Append(@"\trowd");

            for (int y = 1; y <= rows; y++)
            {
                strTable.Append(@"\cellx" + y * csize);
            }

            strTable.Append(@"\intbl \cell \row"); //create the row
        }

        strTable.Append(@"\pard");

        strTable.Append(@"}");

        this.Texto.SelectedRtf = strTable.ToString();
    }

So far so good, but how can I change the size of the table border ?