I assume you know of
Mud Connector? There quite a large list of muds on it, with descriptions about sizes and all...
Personally I can recommend Discworld MUD. If you're into Terry Pratchett you're going to love it. It's quite different from other MUDS and has some nice features. Plus a very large, active playerbase
And, allthough I've never played it,
Wheel of Time MUD has always appealed to me. I'm a bit anxious to start playing since I have a tendency to get addicted to MUDs quite fast which is something I can't use right now
