The pelt of the beast (15,600; 8 lbs.), among other effects, says that
The wearer may take the form of the animal from which the pelt came three times per day as if using beast shape I.
The spell beast shape I says that
When you cast this spell, you can assume the form of any Small or Medium creature of the animal type.
Animals from which the pelt can come include, among others, the Diminutive bat, the Large crocodile, the Tiny rat, the Large shark, the Large tiger, and the bear, which could be Large. (I've provided these animal links as likely animals; precise animals aren't described by the item.)
What happens when a creature uses pelt of the beast to assume the form of a creature that, because of that's creature's size, cannot normally be assumed using the spell beast shape I?