For a small site with not so much traffic, not so much. Your needs in this case can be met with 10 to 100 MB of disk space and 1 to 3 GB of data transfer per month. (Be aware that movie and sound files will eat up a lot of disk space and bandwidth.) If your web site is text-based, you needn't worry about this technical stuff - text takes almost no space at all - not even really big font sizes!
If you're planning to sell something online using our ecommerce commercial web solution or a secure online payment system. Choosing a large web space with a high bandwidth allowance is a good idea in this case.
In this category, 50 MB of disk space or more is advisable. Bandwidth allowance really depends on how many visitors you're expecting and how well you promote your web site. For most sites, 5 to 10 GB should suffice.