Ohalot, Chapter One, Mishnah Seven
Whole limbs [of the body] have no [restriction as to] size: even less than an olive-sized portion of a corpse or less than an olive-sized portion of nevelah (carrion), or less than a lentil-sized portion of a sheretz can defile, [each in the manner of] their respective defilements.
Whole limbs that are separated from either a corpse, or the carrion of an animal, or from a sheretz, always defile even if they don’t meet the minimums for size. These minimums are usually: an olive’s worth for a human corpse, the same size for nevelah (animal carrion), or a lentil-sized piece from a sheretz. The minimum sizes are only relevant when there are pieces that are no full limbs. But if there is a limb that is smaller than that size, it defiles in all cases.