Font size in Pinboard Category pages

You can manually change it by adding a CSS ruleset like this:

.category .entry-summary {
	font-size: 16px;
}

You can duplicate the index.php file and rename it category.php, then you can customize category pages.

The second selector is from the block for which you want to change the font size. I assumed you want to change it for the post content. Alternatively you can just use the .category selector:

.category {
    font-size: 16px;
}