font size of related post headline and title of related posts

Sure, you’ll need to experiment with the sizes and how they do with your theme, but this may help:

#jp-relatedposts h3.jp-relatedposts-headline {
	font-size: 1.5em;
}

#jp-relatedposts .jp-relatedposts-items .jp-relatedposts-post .jp-relatedposts-post-title a {
	font-size: 1.5em;
	line-height: 1.5em;
}

#jp-relatedposts .jp-relatedposts-items p {
    font-size: 1.1em;
	  line-height: 1.3em;
}

Be sure to check out how it looks on mobile as well.

@supernovia Thank you so much for the help, appreciate your efforts and support.
I’ll try it and see how it works. ??

You bet. If it doesn’t work, please include a link to the site so we can see what you see.

@supernovia I put the code above to my function.php and it says there is an error line 19. I am sorry but I am a beginner of WordPress does not know much about these things. please inform me where to place the code. thank you so much for the help.

Oh, I’m sorry. Please don’t put it in function.php; you’ll want to take it out.

This belongs in Additional CSS under Customize.

@supernovia great thank you so much.