The pygmy mouse (_Micromys minutus_) is considered the smallest rat species in the world. It's actually not a true rat, but a member of the mouse family (Muridae).

Here's why it's considered the smallest:

* Size: Adults typically measure only 5-8 cm (2-3 inches) in length, excluding the tail.

* Weight: They weigh a mere 4-12 grams.

However, it's important to note that there are other very small rodent species that could also be considered contenders for the title of "smallest rat," depending on how you define "rat." For example, the Etruscan shrew (_Suncus etruscus_) is the smallest mammal in the world, measuring just 3.6-5.2 cm (1.4-2.0 inches) long.

Ultimately, the "smallest rat" title depends on how you classify the species.