Customize size for H1 only for a particular page (or blog including all posts) without affecting H1-size on other pages

How do I customize the size for Heading 1 on the home page without affecting all other occurences of Heading 1 on other pages?

And how do I change the Heading 1 for a blog including all posts without affecting Heading 1 in other blogs and other pages?

Add to Design > Custom CSS

body.homepage h1 {
	font-size: 20px !important;
}