Sets the size field of a font description, in device units. This is mutually
exclusive with pango_font_description_set_size() which sets the font size
the new size, in Pango units. There are PANGO_SCALE Pango units in one
device unit. For an output backend where a device unit is a pixel, a size
value of 10 * PANGO_SCALE gives a 10 pixel font.