Baranduin does have a point about size. I've participated in probably over 20 RL critique groups--I've lost count. I've seen plenty of types of dysfunction, and I'm sure there's plenty more. The majority of these CGs are failures, where people are more interested in complaining about something or trying to appear clever at the writer's expense than honestly trying to help. Also, critique ability will vary tremendously. You'll get someone who can only do commas-- that's fine, that's where they're at. Then someone else will be good at motivation and plot, etc.

But if you get a CG that works, it's really helpful (_I_ think, which is why I keep seeking them out until I find one that works). I've been in successful groups with 22 members and with 5 members. I think the most common number is 10, but that can get pretty difficult to manage if everybody "brings". Concrit takes time. I personally prefer no more than 8 members, but this is a RL workshop where you sit around and discuss 4-7 ms in an evening.

What the Rocky Mountain Fiction Writers does is spin off numerous CGs so each group has 10-12 authors in it. Then you don't get overwhelmed with ms but you also have enough opinions to see what is generally working or not. I don't have any experience with online groups, so you'll have to take this with a grain of salt. Cheers!