I'm from a small chapter. We only have 7 active member, one marginal member, and 2 inactive members. Next semester we are going to have 14 Members-in-Training so our numbers will finally be up.
I cannot even IMAGINE having 150+ members. I agree with some people in saying that I just can't see that as a true sisterhood; if you don't know some of your sisters, that's a real shame, as where I know all my sisters VERY well. I even know most of their class scedules, what other campus things they're involved in, etc. We're VERY close.
Granted, it's harder to have socials/sisterhood nites when you only have 7 girls, because you have to plan it to where EVERYONE can come. But, if we plan ahead, we're fine. We recently had a Date Night where all our members brought a date to go bowling and to dinner, and it went very well.
A piece of advice for small chapters, especially specialized groups like Sigma Alpha Iota, Tau Beta Sigma, Phi Mu Alpha, etc: why not co-sponsor events with another GLO? We hosted a Halloween Ball with TBS and PMA; we've co-sponsored things with TBS, as well as PMA. It also helps inter-fraternity/sorority relations!
Small chapters aren't too bad. You get to be superclose with your sisters (or brothers), and I think that's the most important thing. I will admit, when we have 20+ members next semester with these new Members-in-Training, it will be great.