You need 6 to start a guild and the raid max is 24. Moorgard said 24 - 40players make an optimum guild if you are trying to acquire prestige.
The more members you have, the more dues, the more ***-kissing you have to do to build/retain city faction/prestige. So bigger is NOT better, in fact, it's harder.
If you are gonna be in a family type guild (ie. non-prestige/iPvP), then it doesn't matter how big you are. My guess is if you're all in the same time zone, at least 50 so you would have enough signed in when you're online to do things. Even more people if you are spread over many time zones and the majority are casual players.
http://www.clanwhitestorm.com/eq2info/
Click on "guilds" at the link, they have a lot of good info.
Edited, Thu Nov 11 14:41:46 2004 by Splatterpunk