Interesting. I am in a guild that has 400+ members. At any one time we have 70-90 people on-line. However, a good 80% of the guild is active (just not all online at the same time). We do raid training, introduction to fractals, strike missions, guild missions, and lots of other events. While the numbers are above the number you gave, I do not consider this a big guild. I consider big guilds to be ViP or HardStuck which actually have multiple in-game guilds.
Having been in six different guilds in my two and a half years of playing Guild Wars I can say that the number of people in a guild is not important. What is important is:
Players active when you are
Content the guild is involved in matches the content you play
Culture of the guild is a good fit
The guild is friendly
Arbitrary numbers do not matter. I hope everyone in this thread can find an on-line home that welcomes and supports them.