I guess you first need to have a dedicated server for optimized CPU and site performance, and provide for at least 20 Mb of space for each user in my opinion. Videos take a lot of space. Or you may want to provide for 5 to 8 Mb per user but limit each memeber's upload file sizes. If you operate a large member hit rate site with shared hosting, members may fall asleep before their page loads. No member stays more than 30 seconds for a page to load before he is gone.