I don't know any huge in-game clan chat's since I barely use them except during wars and events. You can try visit busy runescape worlds such as world 141 or world 1 to find some large in-game clan chats. However, it would be more efficient if you move into the IRC community. Servers such as SwiftIRC has a lot of large runescape clan channels. It's far more efficient than an in-game clan chat since you are able to be in more than one channel at one time. Some official clans such as RSD will also have a large in-game clan chat but I guarantee you that more of their conversations happen in their IRC channel. If you decide to try out IRC, I would recommend you to visit some runescape official clan channels such as #tko_blitz, #cr, #rs-tk, #vr, etc. Those channels will usually be filled and will have endless conversations. I'm thinking that you're trying to find large in-game clan chats to have a warm atmosphere filled with conversations to make some friends. If that's what you're trying to find, then I suggest you to try IRC immediately.