You can have Microsoft Publisher automatically resize your text to fit in a text box.
Turn copyfitting on
After this, whenever you resize the text frame, the font size remains the same.
Turn copyfitting off
If your text is changing size as you type or when you resize a text box, you have AutoFit Text turned on. Here's how to turn it off so that the text stays the same size.
With automatic copyfitting turned off, the font size remains the same whenever you resize the text frame.