In PowerPoint, when you type, you may notice that if you enter more text than can fit inside a text placeholder, the text is automatically resized to a smaller size. This is called AutoFit. In some cases, that might be helpful; in other cases, it might not. Here’s how to turn that off.
The best case for turning off AutoFit is when you know how big you want a box (or any shape) to be on your slide and you know the size you want the text to be. In that case, it makes more sense to let the text overflow the box so that you can edit the text down to a point where it fits.