Weight will vary - what's ok for a maine coon will be overweight for your average moggy! the best way to tell if a cat is overweight is to feel around the ribs. You should be able to feel the ribs but not see them, and there should not be more than a thin layer of fat around them. A clearly defined waist should also be visible.
Neutered cats have a lower metabolism than entire cats (and will be less active) so will need less food, but there's no reason why they will inevitably be overweight.
Obesity is very common in cats, esp those eating dry food. Your vet is the best person to advise you on whether she needs to lose weight. It's dangerous for cats to lose weight too quickly so weight loss is best done under veterinary supervision. If you think she's a bit overweight best to try and get some weight off her before it becomes too much of a problem as it's not easy getting cats to lose weight.
