There is no setting for the font size in this plugin. Hoewever, the font size should scale with the browser controls for zooming (ctrl + or ctrl -). The editor font size is a little smaller than the default font size in the WordPress admin. I think it would be a bit of overkill to add options to such a simple plugin. Would it help to set the defaullt font size to 13px (WordPress admin default) instead of 12px?