If I have a phone with no data plan, and I buy a used iPhone?

When I go into the AT&T store to get my SIM card put in will they MAKE me buy a data plan? Because I don't want one. And do iPhone 4's take the tiny SIM cards or the bigger normal size SIM cards?

Ok so the iPhone 4 takes the tiny SIM card and it is required to have a data plan and if you don't get a data plan then why even buy a phone with no Internet. Waste of money, why would you get a iPhone with no data, then you should of got those original phones 3G with basic talk and text. And it is required to have Internet on the iPhone to activate it.