To find out how much disk space you need, you'll need to learn a little bit about file sizes. If you don't feel like reading a book about various file types and how much space they consume on a web server, then you can simply use your computer's hard drive as a reference.
For example, download a web page from the internet, and then take note of the file size of that web page. Now simply multiply that file size to realize the amount of disk space consumed to host thousands of similar pages. You should also consider multimedia content as well, such as videos and movies. Videos can range in size greatly depending upon whether they are a clip or an entire movie.
It is best to assess your disk space needs before you purchase your web hosting plan by estimating how big your site will be within the foreseeable future. If needed, you can upgrade your hosting plan; however, your goal is to make the site creation process simple and hassle-free. By having a plan with adequate disk space, you remove the need to upgrade unnecessarily.