Chris,
TPS56921 is not in WEBENCH Power Designer currently. You can choose an alternate device in WEBENCH. WEBENCH Power Designer will give you the calculated temperature too, which should help you choose the right device for your application. One device that may fit the bill is TPS54020, which is 10A device (and it is in WEBENCH).
You can also post your question on Non-Isolated DC/DC forum and device experts there might be able to recommend more devices.
Regards