RaTs is a modpack filled with the same or similar mods to the ones used on Rats SMP such as:
Pehkui (For changing size)
Chisels & Bits (For making detailed builds)
Wall Jump (To climb walls)
Farmer's Delight (For added food)
World Edit (To make building/destruction easier)
Biomes O' Plenty (Adds more biomes, plants, flowers, trees and blocks)
Simple Voice Chat (Adds proximity voice chat and group voice chats)
Customizable Player Models (To change your appearance)
For more Pehkui commands visit the mods page, they are listed in its desription, but the main ones you will need to use are:
/scale set <value> [<entity>]
Put the value as whatever you want, 0.22 is the smallest size used in the Rats SMP, the largest being 0.33.
/scale set <scale_type> <value> [<entity>]
You will also need to change your jump height, motion (Speed), reach, entity reach and block reach to suit your new rat size using the scale types:
pehkui:jump_height
pehkui:motion
pehkui:reach
pehkui:entity_reach
pehkui:block_reach