Slice it vertically.
Mike Cohn's SPIDR gives five ways to split a big story into smaller ones that each still deliver something usable.
S
Spike
Split off the research into a timeboxed spike (Day 38).
P
Paths
Split by path: happy path first, alternatives later.
I
Interfaces
Web first, mobile app later. One channel at a time.
D
Data
Support one data type or source first, add others later.
R
Rules
Build the core first, add business rules and limits later.
✕
Not by layer
A “UI story” and a “database story” deliver nothing alone.
Source: Mountain Goat Software: Mike Cohn, 'Five Simple but Powerful Ways to Split User Stories' ↗
Slice vertically. Every slice usable.