If the text box won't resize in Word or Excel, check if the document is opened in the Compatibility Mode, and convert it to a newer format.

Microsoft Office offers an easy way to change default font via settings. Learn how to change the default font in Word, Excel & PowerPoint.

Click "OK" to close the Envelope Options box. The Envelopes and Labels dialog box will show the updated font size on the Delivery Address section and the Return Address.