How to change the width of certain paragraph or header blocks?

• Add a class to the block

• Then you can use css to set width for that block like so:

.your-block-class {
width: 70%;
}

• Add the above css code to your themes custom CSS field or the style.css file in appearance>theme editor. Make sure your using a child theme if you do the later. And of course replace your-block-class above with the class name you created.

EDIT: noticed you did this. I recommend using a % instead of px so it will adapt to mobile, tablets, etc..