Pirate Ship currently offers 4x6" and 2x7" label sizes. To select which format you'd like to use, head to your Settings > General Settings:
The size you select will apply to all label purchases going forward, and you will always need to head back to your Settings to make any changes!
Unfortunately, the size of a label can't be changed after it's been purchased. You would need to request a refund for the incorrectly sized label, adjust your Settings, and start fresh on a new label.