A ver si esto os resuelve las dudas, sacado de su foro (Que por cierto, han limpiado casi por completo):
We want to make sure all of you are ready for the big day. If you don't already know, Black Mesa will require the FREE Source SDK Base 2007 to run. If you are not too sure that you have Source SDK Base 2007 installed or you don't own any source based games, please click here to download and install the Source SDK Base via Steam for FREE.
I know many of you loyal Half Life fans are planning on taking a vacation day from work, calling out sick for school, or even both. We are working very hard to ensure your day off does not go in vain, and that every person trying to download Black Mesa will undergo a smooth and fast downloading experience. The approximate download size will be 3.03 GB, 8 GB uncompressed. At launch, a launch pad will be available offering multiple mirrors and options for download. These options shall be revealed during launch. So go ahead, take that day off and join us!
We know you all are very excited and we want to you share your gameplay experiences as soon as you finish playing. We also want to make sure to collect any bug reporting and get any resolved as quickly as possible. With that being said, the forums shall be moved to it's own separate server. So when you are finished re-visiting Black Mesa, we'll be here to hear all about it.
We are very excited and we are counting down the days right along with you!
- Josh Hubi (Release Personnel)