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.

Learn how to delete a text box without removing text in Word or Excel using Format Shape pane, setting the text box to No Outline, and more.

Right-click the text box and select "Format Shape" from the drop-down menu to change the appearance of the text box. Select "Fill" to change the background color.