Kendra Little explains the rules behind how large a non-clustered index row can be:
So make sure you really need all that junk in your nonclustered index trunk. Er, key.
But even with the expanded size of key columns, sometimes I get asked a question: do columns that “secretly” get added to the key of a nonclustered index count against the maximum allowed nonclustered index key length?
You can read the short answer, but I recommend reading the full post.