In Bill Wake's - INVEST model, which characteristic of a User Story requires that it...

In Bill Wake's INVEST model, the characteristic of a User Story that requires it to be kept to a few person-days or at most, a few person weeks is "Small." This characteristic ensures that user stories are manageable, can be completed within a single iteration, and allows for more accurate estimation and easier testing and integration.

References:

Business Analysis.pdf, "The user story will generally be framed to answer the questions Who? What? Why?... and be expressed in the format: ‘As a {user role} I want {feature} so that I can {reason}.’"​​.