It's 1000 members. And my guild hits that limit all the time. We almost always have a waiting list of people who want to join us. We really have no choice, we have to kick players who have been inactive for quite awhile to make room for players who want to join right now.
Even so, I am not opposed to your suggestion of a 100% renown penalty for kicking members, as long as there is a reasonable time limit after which the penalty goes away. If a player has not logged in at all for more than say 3 months, then the guild should not be penalized for kicking them to make room for a currently active player. The fears of a legion "Krothos Army" guilds destroying all that is good and wholesome about DDO are way overblown, but your suggestion of a 100% kick penalty would help reduce the hysteria some. There is no way you can argue that someone who has not logged in for more than 3 months was somehow being abused because the guild kicked him to make room for a currently active player. You should be able to do that penalty-free.