Estimating in the small is not impossible, and if you are doing sprints you need to know what a sprint's worth of work looks like. But the whole idea of agile

Estimating in the small is not impossible, and if you are doing sprints you need to know what a sprint's worth of work looks like. But the whole idea of agile is that requirements will change during the life cycle of the project and you need to be able to adapt. You'll show the Product Owner your work from the sprint, and they will want to refine the specs, or see potential improvements, or add features. If you sit down with the initial specs and implement them, you will build the wrong thing - so it doesn't matter if you estimate the time correctly.