User cannot change font size only, he needs to set both font and font size with (efl_text_font_font_set)
To change size only, you need to make two calls, one to get font (efl_text_font_font_get) , then pass it again with new size to (efl_text_font_font_set).
If user want to change size only, then he passes NULL as font argument to keep same font.
If user want to change font only, then he passes 0 as font-size argument, to keep same font-size.
This is not best solution, but it better than current behaviour.
I think best solution to have separate function to set font size, but It might break current api or duplicate functions.