Check your theme's css file, which should be called style.css. Find the style that applies to your post(s), which is most likely called ".post", and change the font there. Keep in mind that the stylesheet may be setting the overall site font in the body style (at or very near the top of the stylesheet) and using relative font sizes for individual styles, so you may need to familiarize yourself with the entire stylesheet so that you know what is going on everywhere before you make changes.
keep a backup copy before you make any changes so you can revert if you don't like a change, or multiple changes, that you make.
I generally always work on a "sandbox" file on my local computer first before uploading style changes to my server. You can do this by navigating to a page on your site that has the components for which you want to change the style, select "View Source" (however your browser is set to do that) and then save the source code file to a .html file on your computer, in your theme folder (I use "sandbox.html"). Then open that file in your editor, go to the line near the top that references your stylesheet and remove the URL part of the path, so it just goes to "href="style.css"".
That way you can change your styles and preview it locally - when you're happy with it upload the new stylesheet.