You would do this with CSS, however, a global CSS element for this may have many unintended consequences, as the paragraph element is used in a wide variety of places and by many plugins, over which we have no control. I fear that if you increase the setting on a global level for all paragraphs, you're going to soon discover unwanted font-sizes all over your site in the most unexpected places.
Hence, I'd recommend you create a unique paragraph class and apply the class to the desired paragraphs. This way, only those paragraphs in the class will display that font size and any created without the class will remain as normal. You can find more on doing this here: