web/web_dialog_size/templates/assets.xml

<?xml version="1.0" encoding="UTF-8"?>
<odoo>
    <template id="assets_backend" name="web_dialog_size assets" inherit_id="web.assets_backend">
        <xpath expr="." position="inside">
            <link rel="stylesheet" href="/web_dialog_size/static/src/scss/web_dialog_size.scss"/>
            <script type="text/javascript" src="/web_dialog_size/static/src/js/web_dialog_size.js"></script>
        </xpath>
    </template>
</odoo>