Hi emondoz1,
To do that requires you to add some custom css to your website, as the size of the heading tags are defined by your theme.
You can either edit your style.css directly (not advisable), or edit the style.css of your child theme (better), or use a plugin like My Custom CSS to give you an easy field to insert css code into without messing with your theme.
Each theme will have slightly different requirements:
Here are some related discussions you may find helpful:
https://www.ads-software.com/support/topic/how-to-change-the-font-size-of-a-page-title?replies=11
https://www.ads-software.com/support/topic/how-do-i-increase-header-title-font-size?replies=12
https://smallbusiness.chron.com/change-header-tags-font-size-wordpress-50689.html
~ Kim ~
Support at Yoast
