Post text size
The blog post text size is the default website global font size se in Theme Options > Typography > Advanced > Base Font Size.
However, if you want to customize the posts font size only you can add this code snippet into Theme Options > Layout > Advanced > Custom CSS:
.single-post .entry-content {
font-size: 18px;
}Post Title size
The Blog Title font size uses the default site Heading font size set in Theme Options > Typography > Advanced, however, if you are looking to customize the default Blog posts Title font size only you can use this code snippet into Theme Options > Layout > Advanced > Custom Header Tags Include:
<style>
.single-post .pix-post-title,
.single-post #pix-intro-sliding-text .h3 {
font-size: 24px !important;
}
</style>For detailed information about theme typography you can check this article: