What clothes has Hermione been given to wear?

A very, VERY large robe

It was the largest robe Hermione had ever seen. She marvelled at its size as it flowed through her fingers. These would have been too big even for Crabbe and Goyle!

Still, it was better than going naked.

Hermione shrugged her arms into the sleeves and looked down at herself to assess the damage. She couldn't stop herself from laughing at how utterly ridiculous she looked.

The sleeves were so long they trailed on the floor; she had to roll them 7 or 8 time up to her elbows so that she was able to use her hands. The heavy material sagged off her pale shoulders, forcing her to hold the robe in place with one hand and regularly shrug her shoulders to keep it in place. Hermione had to use her other hand to hold the front of the robe closed; otherwise it would hang open, exposing her breasts and stomach. Unfortunately, this meant that she had no way of hitching up the hem, which trailed behind her like the train of a wedding dress. Her legs and feet were completely enveloped in the fabric, which meant that it would become tangled around her legs and threaten to trip her up once she started walking.

There was no point hanging around there any longer. The sooner she left, the sooner she could get back to her dormitory and put on her own clothes.

Unlocking the cubicle, she shuffled over to the door and peered out. Hermione's heart rose as she realised that her luck might finally be changing.

The distant cheers drifting in through an open window told her that a Quidditch match was in full swing. That meant the school would be almost entirely deserted; but as the length of matches famously varied, Hermione didn't know how long she would have. She wouldn't have a better chance to get back to her dormitory undetected.

Stepping out onto the cold stone floor and gripping her oversized robes tightly, Hermione hurried towards the Grand Staircase, the fastest route up to the seventh floor.