Akeem, I'm a bit confused by your explanation for the problem. What do you mean by saying "site wide"?
The answer to your question depends on what exactly you want to implement on the site.
If you want to enlarge the main title of the blog post, this will be one solution.
If you want to enlarge the subheadings within the post, this will be other solution.
Or you want to enlarge subheadings not only in the post but also in the newsfeed and in the widget and on the blog post listing page.
Or you want to enlarge the main title of the blog post in the newsfeed and in the widget and  on the blog post listing page.

As you see there are many options so please provide us with the screenshots and more details.

Thanks Taissa i would just link to enlarge the main title of the blog post as well as the sub headings withing the post.

Akeem, if I understand you correctly, you want to enlarge the Main title and the subheadings just within the blog post page.
If so, it is possible, but requires custom code modifications within the .php and .html files because there are no custom CSS classes for the titles of the Blog post page.
So you need to paste the custom CSS class for the Main title ( <h1> tag) and for the subheadings (for example <h2> tag).

Go to /ow_plugins/blogs/controllers/view.php file and find the line:
$this->setPageHeadingIconClass('ow_ic_write');

Add custom CSS class for example "blog", so the modified code will be:
$this->setPageHeadingIconClass('ow_ic_write blog');

Then go to /ow_plugins/blogs/views/controllers/view_index.html file find and add the "blog" class to the line:
<div class="ow_superwide" style="float:left;">

the modified code will be: <div class="ow_superwide blog" style="float:left;">

Now we can add CSS attributes for <h1> and <h2> tags using this class. Go to Admin Area>Appearance>Edit Theme>CSS tab and add code:

h1.blog {font-size: ;}
.blog h2 {font-size: ;}

After you have made the changes you should clear the browser cache and enable DEV_MODE(follow the instructions: https://docs.oxwall.org/plugin-tuts:enable-debug) to clear the compiled resources.

However you need to remember that all your customizations will be overwritten while next update of the software, so you need to remember them and apply again, or implement the changes via custom plugin.

If I took you wrong - please provide me with more details.

Taissa Have to say thank you for your simple explanation on what i needed i will now follow your directions.