Being big was useless, and Cassie had never been in a situation like that before. Big didn't mean she was protected from the zombies, it just meant there was more of her to eat. So instead she went small, about as big as a mouse. Small meant she had a better chance of hiding, less of an appetite and- most importantly- was hardly anything worth feeding on. Being small, however, did have the disadvantage that she couldn't get anywhere quickly.
She'd been across the city when the panic first started and it actually took days, changing between miniscule and normal size and hiding constantly and fighting zombies and foraging for food, to make it home to Stark Tower. Except she couldn't even get close. Manhattan (unsurprisingly, this part of town had such a bad track record) was mostly impassable between hordes of zombies, military barricades, and the wreckage of what used to be streets turned war zone. She tried to help others where she could but as the days went on there were fewer people and a lot more undead. Cassie wasn't sure what to do now, she just knew that she needed to survive. That probably meant leaving the city.
She was giving up on actually finding any viable means to do that when she heard a sound that was like music to her ears. A motorcycle. Zombies couldn't ride motorcycles, as far as she knew. Cassie decided it was worth the risk and so she grew to her normal height and darted out into the road, waving her arms wildly while trying to avoid the zombies milling about (and totally attracting their unwanted attention).