Just select square prints. 4" prints are the default size.
Pick your photos, and on the review page, you'll just hit the Super Size Your Prints button to ensure you get the 5.5" squares!
App users that forget to upgrade- Just head to your cart, look for your square prints, and you'll see the Available add-ons option. Click that, and you'll be prompted to upgrade sizes.
Warning, once you've selected 5.5" prints, you cannot revert back to 4" prints.