I don't know how to create an image from RTF, but I do know how to get
the height for a given width. The RichTextBox control has an event
ContentsResized, that is called when the size of the content changes.
The event has an argument of type ContentsResizedEventArgs, that has a
property NewRectangle of type rectangle.

// myRTB is the RichTextBox

myRTB.ContentsResized += new
ContentsResizedEventHandler(myRTB_ContentsResized);

private void myRTB_ContentsResized(object sender,
ContentsResizedEventArgs e)
{
int height = e.NewRectangle.Height; // the new height
}