You are able to do this with a master slide. I believe you may be using them incorrectly. You create a master slide, then fill in the empty text boxes that are created in the slideshow.
At this point, you may be able to resize all of the text by going into outline view, selecting all of the text, and resizing. You should be able to select individual lines of text as well, and adding to the selection by holding CTRL, and selecting more lines. Then you can, for example, resize body text, and header text.
I tried this and it works.