Final answer:
The Binding of Isaac: Rebirth item that gives +0.3 Speed Up, Player Size Down, and a random pill upon pickup and is only found in the boss room pool is Caffeine Pill.
Step-by-step explanation:
The item in The Binding of Isaac: Rebirth that provides the effects of +0.3 Speed Up, Player Size Down, and gives you a random pill when picked up, and is found only in the boss room pool is Caffeine Pill (Option C). When a player picks up Caffeine Pill, it not only increases their movement speed and decreases their size, making them a smaller target for enemies, but it also grants a random pill, which can have a variety of effects. This item is one of the many that add strategic elements to the game, as it affects gameplay directly by altering the character's speed and hitbox.