Shrink.

Shrink is a small mod that lets you change your player's size. By shrinking, you gain access to places that would normally be off-limits without breaking blocks. The mod introduces a single item called the Personal Shrinking Device. When you shift-right-click this device, your player magically shrinks down to about the size of a quarter of a block. The Personal Shrinking Device requires power from any RF/Forge Energy source, although you can disable this feature in the configuration file. Imagine infiltrating secret passages, exploring tiny nooks, or even hiding from mobs as a minuscule adventurer. Need to slip through a narrow gap? Shrink! - Want to experience the world from a bug's perspective? Tired of towering over everything? While small, you'll still pack a punch. Your tiny self can wield mighty weapons and armor. Be cautious some mobs might not appreciate your newfound compactness.