See Amazon's documentation for information on instance sizes.
Note on Heavy reserved instances: We chose to present Heavy reservations as a fixed cost rather than a sloped line because they differ in their billing from other instances—you are charged the hourly rate for use of a Heavy reserved instance whether or not you are using it. With any other reserved instance type, you are charged the up-front fee, plus a monthly bill for however many hours you used. With a Heavy reservation, you are charged an up-front cost, plus a monthly bill for 100% usage, regardless of your actual usage. At the moment you buy a Heavy reserved instance, you know the total cost for that entire contract. To make it easier to tell where break-even points are for usage over long time periods, we're representing Heavy instances as one fixed cost.