You can’t show rulers in Word Online unfortunately. But if you have the Word desktop application, click Open in Word to open the document and see rulers.
To learn more, see Show or hide rulers in Word or Show or hide the rulers in Word 2016 for Mac. If you don’t have the Word desktop application, you can try or buy it in the latest version of Office now.
Tip: Word Online works seamlessly with Word and other Office desktop programs, and we update it based on input like yours. For the latest feature announcements, visit the Office Online blog.