Please find code here :-

file path like:

static/src/xml/editor.xml

editor.xml

<?xml version="1.0" encoding="UTF-8"?>
<template id="template" xml:space="preserve">
<t t-name="web_editor.toolbar" t-extend="web_editor.toolbar">
<t t-jquery='div[id="font-size"] ul' t-operation='append'>
<!-- here set 66px font size.
take data-arg1="66px" -->
<li><a class="dropdown-item custom_font_size" href="#" data-call="setFontSize" data-arg1="66px">66</a></li>
</t>
</t>
</template>

After add this file in module you need to link this file in manifest

like,
'assets': {
'web_editor.assets_wysiwyg': [
'your_module_name/static/src/xml/editor.xml',
],
}