There aren't any specific social network hosting requirements, you will need lots of bandwidth and disk space, and since you expect so many visitors (is it grounded, btw?) CPU usage will be significant, VPS or dedicated server should be your choice.
You can't get the answer how much space you will need, you'll have to answer it yourself, it alll will depend on how popular your site will be and how much will be added by users. To be on the safe side, start with the smallest possible account and upgrade to a bigger one when you need it. Make sure the host you'll choose can provide you with that. My host hqhost.net can help you to choose the config you need and smoothly upgrade you once you grow.