[This is an excerpt from my upcoming book, Membership Site Design.]
A basic WordPress installation, plus some basic text-based content, should take up less than 100 MB of space on your host’s server. However, unless you know you won’t be taking up that much space, I’d recommend going with at least 250 MB of space. This gives your site room to grow, and to store lesson content. I do not recommend going with a host who offers you “unlimited space.” Remember, they’re offering that to each and every one of their other customers too, and some of them are going to take as much space as possible. More importantly, they will also take the computing power of that server, leaving you with a slow-running site.