Joel Spolsky - Evidence Based Scheduling:

"Individual development tasks are easy to estimate, because you’ve written subroutines, created dialogs, and parsed files before.

If you are sloppy, and pick big three-week tasks (e.g., “Implement Ajax photo editor”), then you haven’t thought about what you are going to do. In detail. Step by step. And when you haven’t thought about what you’re going to do, you can’t know how long it will take.

Setting a 16-hour maximum forces you to design the damn feature."