When a big Product Backlog Item (or a User Story) can not be implemented within one Sprint, it's best to split it in to multiple smaller (thin) slices. Then the team can pick up these smaller items one by one for implementation over number of Sprints.
Smaller Product Backlog Items are easier to implement, require less effort, less investment and allow to get early feedback, reducing risk and maximizing learning.
For Example: A brand new ecommerce portal requires ways to accept online payments. The Accept Online Payments Product Backlog Item is pretty big in itself. It can be splitted (sliced) in few ways as below:
- "Accept Online Payments" can be split in to four pieces as below: 1a. Accept Visa, 1b. Accept Paypal, 1c. Accept Mastercard, 1d. Accept AMEX, Discover.
1a. "Accept Visa" can be split in to pieces further: 1aa. Accept Visa Cards issued in USA, 1ab. Accept Visa cards issued in rest of the world.
The PBI 1aa. can be further split based on APIs available from Visa.