magento-test/app/code/Magento/Usps/etc/adminhtml/di.xml

<?xml version="1.0"?>
<!--
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
-->
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Shipping\Block\Adminhtml\Order\Packaging">
        <arguments>
            <argument name="sourceSizeModel" xsi:type="object">Magento\Usps\Model\Source\Size</argument>
        </arguments>
        <plugin name="usps" type="Magento\Usps\Block\Adminhtml\Order\Packaging\Plugin\DisplayGirth"/>
    </type>
</config>