It depends on your cat’s size, weight and nerves. Weigh your cat and choose the backpack with similar carrying capacity. Make sure your cat has enough space especially if she is a nervous little fellow or if you combine the safety of the backpack with a leash and harness.
Your cat should always have enough space to lie, stand, turn around and have a snack. Even if you own a kitty, you can get her a grown-up cat carrier. Kittens grow at incredible speed and will reach their full size pretty fast. Until they do, fill in the extra space with a towel or an additional blanket.