How much storage do you need for a multi-media website?

It depends on how you deploy your site. It is recommended that video and image files be stored outside of your database. That said this is more a question of how much hard drive space you need. Your database size will be influenced by how the database is built, the functionality of the application which speaks to how much detail is maintained in the database, and how many records (number of customers as an example) you will have. There will also be credit card transactions that need to be maintained in a database for years, pointers to your content which should be database driven, and other factors. In short it is not possible to blindly answer this question with any level of accuracy.