Sets the PGPLOT character size in world co-ordinates
Note SGS and PGPLOT behave differently if the scales on the X and Y axes are not equal. SGS keeps the character size constant in world oordinates, so absolute character size will be different for vertical and horizontal text. On the other hand, PGPLOT keeps the absolute character size fixed, resulting in the characters size in world co-ordinates varying for horizontal and vertical text. This routine sets the size for horizontal text. If the axis scales are not equal, vertical text will have have a different size (in world co-ordinates).