Level: Druid 2, Ranger 3
Components: V, S
Casting Time: 1 standard action
Range: Touch
Target: One willing animal of Small, Medium, Large, or Huge size
Saving Throw: None
Spell Resistance: No
This spell causes instant diminution of a single willing animal. (A willing animal is not one with which you are engaged in combat, for instance.) This halves its height, length, and width and divides its weight by 8. This decrease changes the creature’s size category to the next smaller one, allowing it to fit better into tight spaces, such as the typical dungeon room or subterranean passage. The animal gains a +2 size bonus to Dexterity, a –2 size penalty to Strength (to a minimum of 1), and a +1 bonus on attack rolls and AC due to its reduced size.
A Small animal whose size decreases to Tiny has a space of 2½ feet and a natural reach of 0 feet (meaning that it must enter an opponent’s square to attack). A Large animal whose size decreases to Medium has a space of 5 feet and a natural reach of 5 feet. This spell doesn’t change the target’s speed.
All equipment worn or carried by a creature is similarly reduced by the spell. The animal’s natural attacks deal less damage (see Decreasing Weapon Damage by Size). Other magical properties are not affected by this spell. Any reduced item that leaves the reduced animal’s possession instantly returns to its normal size. This means that thrown weapons (say, hurled quills from a giant porcupine) deal their normal damage.
Multiple magical effects that reduce size do not stack, which means (among other things) that you can’t use a second casting of this spell to further reduce the size of an animal that’s still under the effect of the first casting.
Source: Player’s Handbook (version 3.5), page 269.