The ability to resize fonts in atto has been removed from core. It should be put back into core. However, there is a bug in the existing atto font-resize plugin in the plugins database. The <font> tag is stripped from the text by core editor postprocessing and this makes the plugin non functional. This has been reported previously as part of MDL-48121 which was closed. However, the problem remains.

https://moodle.org/plugins/view/atto_fontsize

The plugin creates a button in atto that resizes highlighted text in the editor. However, upon saving, all changes to font sizes are lost. This appears to be due to atto stripping the deprecated <font> tags from the html upon saving. I propose that the plugin in the database be updated with code that changes the CSS values within <span> tags. The proposed replacement plugin can be found at: https://github.com/adamjenkins/moodle-atto_cssfontsize

Either this plugin should be included as core in the future or at least the old non-functioning plugin should be removed from the plugins database.

See also the discussion at: https://moodle.org/mod/forum/discuss.php?d=311302