Some good advice from Mike Cohn on how to approach story decomposition:
When something will take a long time to develop, that's exactly when I want to use a process that forces me to get early and frequent feedback. This will counter many developers’ tendency (including my own) to retreat into a cave thinking we know what our users want.
In my experience, it's always worth digging a little deeper during planning to find ways to split user stories so they can be delivered in a single sprint.