This happens to me too. I never thought of it as a "bug" really, but it is a bit strange. The ending text size (once the text is placed on your map and you "leave the box") is the proper size that you set in the Font size number. In the interim, while you have the Text box open and you've already typed a word or letters, and you change the font point-size on the tool, it will suddenly change the text in your text box to an outrageous size. But when you actually "place" the text box and drop it on the map, and you let go of the box, then it will revert to the expected text size. So for me it is a temporary glitch, that occurs only while you have a text box with words & you change the font point size while holding the text box open.
