Atto font size is a plugin for the atto text editor in Moodle which allows you to alter the font sizes using the Atto text editor. In the default moodle installation with atto editor, you can’t change the font size of the text, but this plugin enables you to change it.
You need to install this plugin in your_moodle_site\lib\editor\atto\plugins\ directory and visit the notifications page to complete the install process.
Afterwards you need to add the fontsize plugin to your atto toolbar settings page from Site administration> Plugins > Text Editors > Atto HTML editor > Atto toolbar settings in the Toolbar config field in the section where you want to have the fontsize option.
You can downlaod the Atto font size plugin from this link: https://moodle.org/plugins/view/atto_fontsize