Most developers have a intuitive understanding of what a large refactoring is. But it is difficult to come up with a simple definition.
Therefore there was a workshop on large refactorings at OT2003Conference. The results and a place for a wiki discussion are available:
-- StefanRoock
Also see BigRefactorings and BigRefactoringsAreHard.
External references:
Also see NoseJobRefactoring. Could we conclude that all nose-jobs are "big"? It isn't obviously true that all BigRefactorings are nose-jobs.