Why does it work best for small tasks? His example tasks are 5-8 pomodoros long.

In my opinion, no self-help guide applies for everyone; we all have our own strengths and vices.

It seems that this one is designed to help build concentration and minimize interruptions by defining concrete "work unit" blocks, thus helping to build discipline in fighting off distractions (internal & external).

I don't feel strongly about his views on the perception of time, but it's not that important to the whole article.