The SyntaxHighlighter Evolved plugin is great, but there is no way to adjust the font size! Here is what worked for me.
From your WordPress admin panel, open Appearance » Customize.
Then, on the Customize page, click on Additional CSS.
In the little Additional CSS box we’re going to restyle SyntaxHighlighter and make the font size 75% of our normal blog font size (set the percentage to whatever works for you, of course).
div.wp-block-syntaxhighlighter-code {
font-size: 75% !important;
}
Be sure to click Publish, and you’re done!