You are nearly there! You see the line in the item.html.twig that reads:
{% include 'partials/blog_item.html.twig' with {'truncate':false,'big_header':true} %}
This is passing a variable called big_header to your blog_item.html.twig template. Depending on the behavior you want, you either need this to be false by default and enable it when needed, or true by default and disable it when needed.
We'll assume on by default, and disable when needed in this example. Then you just need to add a custom header to your item.md page file:
and in your item.html.twig file, change the include:
{% include 'partials/blog_item.html.twig' with {'truncate':false,'big_header':page.header.big_header|default(true)} %}
Not tested, but should work!
BTW, if you start modifying the templates in the Antimatter theme, you should make a copy of the theme with your own name, That way if you update Antimatter via GPM, you can manually diff/merge against your custom theme, and not lose anything.