After you purchase a label on Shopify, please click on "Change Format" on the right-hand side and choose 4 x 6.
🌟 Note
Shopify Shipping is available for orders shipped from fulfillment locations based in the United States, Canada, Australia, and the United Kingdom.
If you use Shopify Shipping, then you can buy shipping labels when you're fulfilling an order in Shopify. If your fulfillment locations aren't based in the United States, Canada, or Australia, then use a shipping label app instead.
Before you purchase labels, make sure your Shopify account is set up for Shopify Shipping.
After you buy a label, you can print or void it from the order page.
Shopify official explanation: https://help.shopify.com/en/manual/shipping/shopify-shipping/shipping-labels