1. There are different kinds, the Ramshorns or Marisas get maybe the size of a quarter, P. Bridgesiis (aka mystery snails, the most common kind) MAY grow to 2", and Canas can get the size of a baseball.
2. Being vegetarians, they make a lot of waste. I'd recommend a filter but it isn't absolutely needed if you'd rather not. They don't need extra oxygen, having both a lung and a gill.
3. Maria (known as mariastuff on applesnail.net and petlady5 I believe other places) makes good food and treats for them. They like different vegetables, I feed them carrots. Just plop one in until they finish it.
4.

No. Snails only love bite. Hehe.. they don't bite like a mammal would, they rasp their tounge (with teeth on it) against your palm. It feels like a miniature cat licking you. It's quite calming. Obviously I'm a snail nut.