When I’m reading a story and I see 2 pieces of acceptance criteria, when I do this, it does that AND when I do this other thing, it does this other thing. That is often a good candidate for a story to be split. Also, overly large stories are harder to test and accept because there are too many variables. Keep it simple and deliver.

Each rule is small enough, but in total they all will make up to a substantial amount of time to develop. That will, in turn, require a good amount of testing, as well.

Consider Main Goal of User Story. Split Story to Achive That Goal Faster

From the same post as above but here’s a handy chart to help split user stories in the most logical way.

This is great, big projects feel like you’re Wil E. Coyote running off a cliff, a whole bunch of running in place until you fall.

 
So Agile doesn’t scale, and that’s a good thing. It’s not supposed to scale. It’s supposed to help us realise that we need to scale down. Split stuff into smaller parts. Build a prototype. Deliver frequently. Get feedback from real users quickly. You know the drill.