To update your Facebook Marketplace settings so it generated 4x6 shipping labels, here is the method:

1) Sign in to Facebook;

2) Go to Marketplace, then click "Your Account" icon;

3) Click "View Order"

4) Change the label size by clicking "Change"---"Change label type";

5)  Select Format "4x6 in. Label";

6) Finally, select "Create Delivery Label"