Theme Horse

Hi jack3,

If you are using WordPress 5+ version then you can use the block feature of WordPress and use the Paragraph Block through which you can easily increase the font size of paragraphs to any desirable sizes. Even you can set different font sizes for different paragraphs.
But if you are using WordPress below 5 Version then you can only increase the font size of content which you can find on our Theme Options on customizer. (it will change the content font size of your entire Site).
Still to change the font Size of content for only Page/post Single page then please go to ‘Appearance > Customize > Additional CSS’ and paste the below Custom CSS Code.

.page-template-default .site-main .page,
.page-template-default .site-main .hentry,
.post-template-default .site-main .post,
.post-template-default .site-main .hentry {
	font-size: 17px;
}

Thanks