It's a manual process to change the starting sizes for the files. SQL won't do it.

TempDB data files should all have the same initial size and same autogrow...

Leave it to run please. We need an idea of how many rows are going to have to move in order to decide on a strategy

And I didn't...

If the split takes time then there's data that needs to be moved to the new partition. It's generally recommended to split so that the new partition is empty. If...

Then I'm afraid you'll have to live with the deadlocks.

For anyone curious, what's happening is the following:
Session 1 (the update) takes a U lock on a key....

Name on the BEGIN TRANSACTION - fine. Name on the COMMIT TRANSACTION - fine. Name on the ROLLBACK TRANSACTION - potential error that can leave orphaned transactions.

Ok, bit of a short reply because I need to leave work 10 minutes ago to miss the traffic...

Partitioning is NOT for performance. It's for data management, for...

GRANT SELECT, INSERT, UPDATE, DELETE, EXECUTE ON schema::Sales TO <database role>

That's the basic principal of it. Don't grant permissions to the database, don't use the db_owner fixed role...