the very first thing that should have been done is write a spike card or cards to find ways to break that down to digestible components.
But really, why bother? Why not just say "Well, this is a large task - prolly take a few weeks. Let's get going." How does time spent breaking things down into small bits ahead of time actually help anything beyond allowing you to play the game of "fit the task into the sprint so we can get on with the work"? With or without a sprint-based method, you'll naturally break stuff down as you get into the task anyway.
I dunno. Just seems like weird voodoo to me.