Depending on whether you're a blogger or a multinational corporation you'll need to know how much bandwidth and total storage space you're going to need to comfortably support your site. A small blog is not going to need at 1TB/s bandwidth cap and 600 GB of storage space on the server. Think realistically about how much traffic is expected and get a plan that supports it. Most web hosting companies make it easy to upgrade and will (happily) do so when you feel it's necessary.