The only reason I asked for the largest was in hopes that I'd find the guilds who actually have a decent number of people online at the same time. One of my alts is in a casual guild right now. The people are super friendly and I think they raid form time to time. But whenever I log on, there is no one on. Kinda of makes me sad.
Having 15+ people on during EST prime time is plenty as far as things go for a fun, casual atmosphere.
If the guild also has a cross faction sister guild, that's even better. We have alts of both factions.