The
Height
and
Width
properties of RichTextBox control can be set to the required values, say in the Click event of a Button control.
RichTextBox1.Width= 500;
RichTextBox1.Height = 300;
RichTextBox1.Size=new System.Drawing.Size(500,300);
However it will not work if the Dock property of the RichTextBox is set to a value other than None.