This is controlled by your theme's css and currently this is not customizable directly within the side editing panel within Gutenberg but can be updated by updating your theme's CSS. 

The exact css selectors may slightly change for each theme but the following 

    .entry .entry-content .wp-block-quote p {
      font-size: 3px;
      font-style: normal;
      line-height: 1.8;
    }

Note that if you place inside your theme's CSS file and update your theme, the change will be lost. 
so you can either make a [child theme][1] or save this somewhere else and then if your theme is updated, copy this back in. 


  [1]: https://developer.wordpress.org/themes/advanced-topics/child-themes/