Hey there marchnall,
This can be done by adding some custom CSS. The easiest way to do that, if your theme doesn’t already have custom CSS tab, would be to add it in your site using the following plugin
https://www.ads-software.com/plugins/simple-custom-css
Once the plugin is installed and activated you can add this code to Appearance >> Custom CSS:
.entry-content {
font-size: 1.25em;
}
Increase/decrease the numeric value to what ever suits you the most. This should affect the font size in pages, posts, single post pages and category page.
Hope this helps.
Best regards,
Bojan