Don't worry about it; whatever they offer should be more than enough for most businesses.
If you need more -- you put up tons of product pictures, or decide to offer video tours, or get into e-commerce and catalogs and other options that eat up a lot of disk space -- they'll sell you more. Check the hosting service's menu of prices to see how much more it will cost to, say, double the basic offering. Consider that your upside risk of future success.
If you don't like that answer, or you are going for a more ambitious site, here are some rough-and-ready approaches to figuring out your hard drive space needs:
These methods aren't bulletproof, but they will give you a start at gauging which of the many packages offered you ought to buy.
Since the Web hosting service pays for access to the Internet, every hosting service has some upper limit on the amount of bandwidth you can use, given as the number of gigabytes downloaded from your site each month by your visitors. Generally you get plenty of bandwidth, especially for a basic business Web site.
If for some reason you become the hot new pancake house, mentioned on national TV or involved in some improbable scandal, and everybody rushes to see your site, you might run out of bandwidth and have to pay to get more. But not likely. There is no easy way to calculate in advance how much bandwidth you'll use, but again, the Web host will sell you more if you need it.