In a large group, people will drop out. Absolutely. This is good and bad news: it's bad because obviously the more the merrier and you probably spent some time pre-IC planning stuff that isn't applicable with a different cast than you expected, but it's good because, frankly, you have less people to deal with.
You should basically allow for this. Allow no one character to be utterly indispensable (unless they can 100% be relied on, on pain of death, to stick with it) and don't cement any plots too far in advance. It's fine to pencil stuff in and make adjustments as things change, but nine times out of ten, roleplayers won't do what you expect them to, and nine times out of ten, you won't have all the players you expected by the time you hit the IC.
Chat rooms are a good way to keep many people involved, but this comes at the cost of people who don't share the most common timezone or who have as much free time - it's very easy for such people to get left behind in the frantic, excitable group natter that happens in chat rooms. These are people who're likely to feel like they have to 'catch up' and alienated. At best, it'll mean their characters are likely to be slightly disconnected from events that have been so meticulously planned. At worst, they're people who'll drop out. I would recommend trying to encourage as much OoC discussion as possible on the forum so, at the very least, there's a paper trail. Of course, if people want to work out specific things between themselves, then, by all means, chat rooms are an effective way to go, but information and discussion that involves everybody should be available to everybody.
My main horror in basically all RP's is the Mass Unguided Conversation of Doom - where a tonne of characters are in the same place and attempt to have an unstructured conversation. In my experience, just like chatrooms, this'll typically involve a load of waiting for people in awkward timezones/unsociable hours to catch up, or, more likely, their characters will be forced to be oddly silent during the proceedings that happen without them. If you have a super-good group that can all be available at the same time, then this isn't (such) a problem, but this is unlikely, so I recommend doing all you can to avoid MUCoDs. Depending on what your game is, I'd suggest trying to split your group into smaller groups and allow them to pursue linked but ultimately independent plots and then find a reason to shake it up when things start to get stale.
For me, character sheets are super valuable, and if I'm running a larger game, I like to insist on the importance of clear, easily-read sheets so that you can easily understand and compare information from them. In a game I'm running now, because the players have all submitted sheets in roughly the same format, it's very easy to get a grip on who the characters are and how they're likely to interact and knowing that information will help to steer proceedings.
