How to change blog post body font size

With browser inspection, it shows the default blog content font is 10pt
<div class="post-body'>
    <p>
         <span style="font-size:10pt;">....

I changed the styles.css under the theme. and set
.post-body span {
    font-size: 18px;
}

However, the blog font is still showing as 10pt, my change of font to 18px got picked up, but overwritten by this 10pt. Does anyone know where to change this default 10pt font-size?

Because inline css (here <span style="font-size:10pt;">) has the highest priority. You need to remove inline css.

Go to the blog detail page in the admin panel and set /customize the font size using the rich editor.

Or, you can use "!important" rule after your css statement in order to override the inline css. This way you will keep the original css in case you could need it later. Of course, if you are positive that you will never need that, it is better to edit or remove the original code than overriding it.

Regards
/ Hristo