It can be your hosting server or your web browser cache issue. In this issue, you need to remove/clear your hosting server & web browser cache. After then, you need to reload the web page with Ctrl+F5 to check the changes.
By the way, there have some sections in the template with specific font-size that are not related to GENERAL STYLES. In that case, you need to change those specific section’s font-size.
Please check out the [TABLE OF CONTENTS] to find out the specific section’s styles in the style.css file to change the font-size.