There was a terrible snarling noise. Lupin's head was lengthening. So was his body. His shoulders were hunching. Hair was sprouting visibly on his face and hands, which were curling into clawed paws. Crookshanks's hair was on end again; he was backing away -- 

As the werewolf reared, snapping its long jaws, Sirius disappeared from Harry's side. He had transformed. The enormous, bearlike dog bounded forward. As the werewolf wrenched itself free of the manacle binding it, the dog seized it about the neck and pulled it backward, away from Ron and Pettigrew. They were locked, jaw to jaw, claws ripping at each other. 

"Shit," cursed Amelia. "That idiot is going to get himself killed,"

Amelia turned on the spot and started yelling waving her arms in the air. Sirius and Lupin had stopped fighting, but Lupin's attention was now on Amelia.

"This better fucking work," she mumbled. "Hay teach! Pick on someone your own size!"

Lupin snarled, running towards her.

Ninty feet.

She fell to the ground in pain as her bones twisted and shattered, her spine contorting and bending unnaturally under her skin, which burned like fire as fur covered her limbs. Claws grew out from her nails, ears sprouting painfully through her head. Her muscles cramped, causing her limbs to tremble under her.

Fifty feet.

She bit back a scream of pain.

Thirty feet.

Shapeshifting into larger animles without practices could be extremely painful and even dangerous.

Ten feet.

Standing on her hind legs, Amelia had fully transformed into a werewolf. Lupin lunged at her with a snarl, nearly knocking her over. Despite her werewolf form, Amelia remained smaller than her adult counterpart. Lupin attacked relentlessly, snapping his jaws and clawing. Amelia grasped his shoulders, forcing him back just far enough to avoid his bites. With all the strength she could muster, Amelia threw Lupin away from her. Falling to the ground, he swiped at her, scratching her upper arm.